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Summary of Significant Accounting Policies - Adjustment to Prior Period Financial Statements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued liabilities	$ 15,034	$ 721	$ 9,448
Restatement Adjustment \| Correction Of Provisions For Returns From Customers
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued liabilities			$ 1,860

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 39,181	$ 24,990
Short-term investments	51,508	0
Accounts receivable, net of allowances of $45 and $22 at December 31, 2019 and 2018, respectively	8,601	6,382
Inventories	10,322	5,744
Prepaid expenses and other current assets	2,878	1,408
Total current assets	112,490	38,524
Long-term investments	18,224	0
Property and equipment, net	2,734	2,880
Restricted cash	310	310
Other non-current assets	3,644	1,029
Total assets	137,402	42,743
Current liabilities:
Accounts payable	1,898	1,252
Accrued liabilities	15,034	9,448
Total current liabilities	16,932	10,700
Long-term debt	44,879	44,201
Redeemable convertible preferred stock warrant liability	0	16,091
Other liabilities	3,700	1,069
Total liabilities	65,511	72,061
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock issuable in series, $0.001 par value
Liquidation preference: $121,144 at December 31, 2017 and 2018	0	105,235
Preferred stock, $0.001 par value
Shares authorized: 5,000,000 and none at December 31, 2019 and 2018, respectively	0	0
Shares issued and outstanding: 31,255,267 and 1,135,310 at December 31, 2019 and 2018, respectively	31	1
Additional paid-in capital	263,384	4,557
Accumulated other comprehensive income	2	0
Accumulated deficit	(191,526)	(139,111)
Total stockholders’ equity (deficit)	71,891	(134,553)
Total liabilities and stockholders’ equity (deficit)	$ 137,402	$ 42,743

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock IPO	Common Stock	Common Stock IPO	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income	Non-controlling Interest
Temporary equity, beginning balance at Dec. 31, 2017			$ 105,235
Temporary equity, beginning balance (in shares) at Dec. 31, 2017			21,233,190
Temporary equity, ending balance at Dec. 31, 2018	$ 105,235		$ 105,235
Temporary equity, ending balance (in shares) at Dec. 31, 2018	21,233,190		21,233,190
Beginning balance (in shares) at Dec. 31, 2017					663,270
Beginning balance at Dec. 31, 2017	$ (98,578)				$ 1		$ 2,977	$ (101,556)	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 656						656
Exercise of stock options (in shares)	438,578				438,578
Employee stock-based compensation	$ 728						728
Nonemployee stock-based compensation	183						183
NeuroCo common stock issuance	1									1
Issuance of common stock in connection with NeuroCo merger (in shares)					33,462
Issuance of common stock in connection with NeuroCo merger	0
Net loss	(37,630)							(37,629)		(1)
Comprehensive loss	(37,630)							(37,629)		(1)
Unrealized gain on investments, net	0
Ending balance at Dec. 31, 2018	(134,553)				$ 1		4,557	(139,111)	0	0
Ending balance (in shares) at Dec. 31, 2018					1,135,310
Temporary Equity [Abstract]
Exercise of warrants	31	$ 0	$ 1,784	$ 37,121			31
Exercise of warrants (in shares)			292,361	1,653,004	3,764	2,204
Temporary equity, ending balance at Dec. 31, 2019	$ 0		$ 0
Temporary equity, ending balance (in shares) at Dec. 31, 2019	0		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 1,542				$ 1		1,541
Exercise of stock options (in shares)	873,786				873,786
Issuance of common stock in connection with IPO, net of underwriting discount, commissions and offering costs of $2,481	$ 109,119				$ 6		109,113
Issuance of common stock in connection with IPO, net of underwriting discount, commissions and offering costs (in shares)					6,000,000
Conversion of preferred stock to common stock upon IPO (in shares)			(23,178,555)		23,178,555
Conversion of preferred stock to common stock upon IPO	144,140		$ (144,140)		$ 23		144,117
Issuance of common stock under employee stock purchase plan	1,048						1,048
Issuance of common stock under employee stock purchase plan (in shares)					61,648
Employee stock-based compensation	2,887						2,887
Nonemployee stock-based compensation	90						90
Net loss	(52,415)							(52,415)
Unrealized gain on investments, net	2								2
Ending balance at Dec. 31, 2019	$ 71,891				$ 31		$ 263,384	$ (191,526)	$ 2	$ 0
Ending balance (in shares) at Dec. 31, 2019					31,255,267

Fair Value Measurements - Changes in Redeemable Convertible Warrant Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 16,091	$ 4,185
Change in fair value recorded in other income (expense), net	21,030	(11,906)
Reclassification upon IPO	(37,121)
Ending balance	$ 0	$ 16,091

Stock Option Plans - Activity Under Compensation Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares Available for Grant
Beginning balance (in shares)	57,889	328,290
Authorized (in shares)	2,317,000	223,664
Options granted (in shares)	(848,023)	(629,716)
Options cancelled (in shares)	(27,824)	(135,651)
Ending balance (in shares)	1,554,690	57,889	328,290
Number of Shares
Beginning balance (in shares)	4,364,377	4,308,890
Options granted (in shares)	(848,023)	(629,716)
Options exercised (in shares)	(873,786)	(438,578)
Options cancelled (in shares)	(27,824)	(135,651)
Ending balance (in shares)	4,310,790	4,364,377	4,308,890
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 3.79	$ 3.09
Options granted (in USD per share)	22.77	6.51
Options exercised (in USD per share)	1.77	1.50
Options cancelled (in USD per share)	8.71	1.56
Ending balance (in USD per share)	$ 7.91	$ 3.79	$ 3.09
Weighted Average Remaining Contractual Term (in Years), awards outstanding	7 years 3 months 7 days	7 years 4 months 9 days	7 years 9 months 21 days
Aggregate Intrinsic Value, awards outstanding	$ 140,234	$ 33,132	$ 5,073
Number of Shares, vested and exercisable (in shares)	2,514,891
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 4.27
Weighted Average Remaining Contractual Term (in Years), vested and exercisable	6 years 4 months 17 days
Aggregate Intrinsic Value, vested and exercisable	$ 90,854
Number of Shares, vested and expect to vest (in shares)	4,310,790
Weighted Average Exercise Price, vested and expected to vest (in dollars per share)	$ 7.91
Weighted Average Remaining Contractual Term (in Years), vested and expected to vest	7 years 3 months 7 days
Aggregate Intrinsic Value, vested and expected to vest	$ 140,234

Formation and Business of the Company	12 Months Ended
Formation and Business of the Company	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business of the Company	Formation and Business of the Company The Company Silk Road Medical, Inc. (the “Company”) was incorporated in the state of Delaware on March 21, 2007. The Company has developed a technologically advanced, minimally-invasive solution for patients with carotid artery disease who are at risk for stroke. The Company’s portfolio of TCAR products enable a new procedure, referred to as transcarotid artery revascularization, or TCAR, that combines the benefits of endovascular techniques and surgical principles. The Company’s manufactures and sells in the United States its portfolio of TCAR products which are designed to provide direct access to the carotid artery, effective reduction in stroke risk throughout the procedure, and long-term restraint of carotid plaque. The Company commercialized its products in the United States in April 2016. Liquidity In the course of its activities, the Company has incurred losses and negative cash flows from operations since its inception. As of December 31, 2019, the Company had an accumulated deficit of $191,526,000. The Company expects to incur losses for the foreseeable future. The Company believes that its cash and cash equivalents of $39,181,000 and available-for-sale investments of $69,732,000 at December 31, 2019, as well as its expected revenues will provide sufficient funds to allow the Company to fund its planned current operations for the next twelve months from the issuance of these financial statements. Reverse Stock Split On March 13, 2019, the Company’s Board of Directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.7 reverse stock split of the Company’s common stock and redeemable convertible preferred stock to be consummated prior to the effectiveness of the Company’s planned initial public offering (“IPO”). The reverse stock split was effected on March 27, 2019. The par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All the common stock, redeemable convertible preferred stock, stock options and warrants, and related per share amounts in the financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split. Public Offerings In April 2019, the Company issued and sold 6,000,000 shares of its common stock in its IPO at a public offering price of $20.00 per share, for net proceeds of approximately $109,119,000 after deducting underwriting discounts and commissions of approximately $8,400,000 and expenses of approximately $2,481,000. Upon the closing of the IPO, all shares of redeemable convertible preferred stock then outstanding converted into shares of common stock and the Company's outstanding warrants to purchase shares of common and redeemable convertible preferred stock were exercised, or automatically net exercised absent a prior election. The exercises resulted in the reclassification of the fair value of the related redeemable convertible preferred stock warrant liability to additional paid-in capital. In August 2019, the Company completed a secondary public offering of 4,200,000 shares of its common stock sold by certain selling stockholders, and the exercise in full of the underwriters' option to purchase 630,000 additional shares of its common stock from certain selling stockholders, at a public offering price of $39.50 per share. The Company did not receive any of the proceeds from the sale of the shares of its common stock from the selling stockholders.

Commitments and Contingencies - Purchase Obligation Narrative (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Purchase obligation	$ 7,097,000

Income Taxes - Reconciliation of U.S Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	$ (11,007)	$ (7,902)
State taxes, net of federal benefit	(2,270)	(1,582)
Permanent differences	(4,731)	289
Loss on Series C warrant liability	5,330	2,500
Change in valuation allowance	12,797	6,197
General business credits	(319)	136
Other	208	376
Provision for income taxes	$ 8	$ 14

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock

The Company had the following redeemable convertible preferred stock issued and outstanding at December 31, 2018 (in thousands except share amounts):


	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

Upon the closing of the IPO, all shares of redeemable convertible preferred stock then outstanding converted into shares of common stock. As of December 31, 2019, the Company does not have any redeemable convertible preferred stock issued or outstanding.

Redeemable Convertible Preferred Stock Warrants

Upon the closing of the IPO, all of the outstanding redeemable convertible preferred stock warrants were exercised, or net exercised based on the IPO price of $20.00 per share, into 1,945,365 shares of common stock. As of December 31, 2019 and 2018, warrants to purchase an aggregate of 0 and 2,672,502, respectively, shares of Series C redeemable convertible preferred stock were outstanding.

Stockholders' Equity (Deficit)

Preferred Stock

At December 31, 2019, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 5,000,000 shares of preferred stock with $0.001 par value per share, of which no shares were issued and outstanding.

Common Stock

At December 31, 2019, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 100,000,000 shares of common stock with $0.001 par value per share, of which 31,255,267 shares were issued and outstanding. The holders of common stock are also entitled to receive dividends whenever funds are legally available, when and if declared by the Board of Directors. As of December 31, 2019, no dividends have been declared to date. Each share of common stock is entitled to one vote.

At December 31, 2019 and 2018, the Company had reserved common stock for future issuances as follows:


	December 31,
	2019	2018
Conversion of Series A redeemable convertible preferred stock	—	1,629,629
Conversion of Series A-1 redeemable convertible preferred stock	—	1,111,111
Conversion of Series B redeemable convertible preferred stock	—	6,264,470
Conversion of Series C redeemable convertible preferred stock and warrants	—	15,064,405
Exercise of options under stock plan	4,310,790	4,364,377
Issuance of options under stock plan	1,554,690	57,889
Issuance of common stock under employee stock purchase plan	372,352	—
Warrants to purchase common stock	—	7,527
	6,237,832	28,499,408

Common Stock Warrants

In connection with the IPO, the common stock warrants were cash, or net exercised based on the IPO price of $20.00 per share into 5,968 shares of common stock. As of December 31, 2019 and 2018, warrants to purchase an aggregate of 0 and 7,527 shares of common stock were outstanding.

Commitments and Contingencies - Operating Lease Maturities (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2020	$ 1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,230
Less: imputed interest	(761)
Present value of lease liabilities	$ 4,469

Subsequent Events - Narrative (Details) - $ / shares	1 Months Ended	12 Months Ended
Subsequent Events - Narrative (Details) - $ / shares	Feb. 29, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 31, 2020	Mar. 31, 2019
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)		848,023	629,716
Exercise price (in USD per share)		$ 22.77	$ 6.51
Common stock reserved for future issuance (in shares)		6,237,832	28,499,408
2019 ESPP
Subsequent Event [Line Items]
Common stock reserved for future issuance (in shares)		372,352,000			434,000
2019 Stock Option Plan
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)		2,366,251			2,317,000
Subsequent Event \| 2019 ESPP
Subsequent Event [Line Items]
Common stock reserved for future issuance (in shares)				312,552
Subsequent Event \| 2019 Stock Option Plan
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)				1,250,210
Shares of common stock reserved for issuance (in shares)	92,885
Exercise price (in USD per share)	$ 47.20

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at the beginning of year	$ 1,348	$ 615
Increases related to current years’ tax positions	88	118
Increases/(decreases) related to prior years’ tax positions	0	615
Balance at end of year	$ 1,436	$ 1,348

Balance Sheet Components - Fair Value of Available-For-Sale Investments (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 104,093
Gross Unrealized Gains	9
Gross Unrealized Losses	(7)
Estimated Fair Value	104,095
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,363
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	34,363
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,919
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	9,919
Corporate bonds/notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,180
Gross Unrealized Gains	0
Gross Unrealized Losses	(4)
Estimated Fair Value	10,176
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	44,450
Gross Unrealized Gains	9
Gross Unrealized Losses	(3)
Estimated Fair Value	44,456
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,181
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	5,181
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	34,363
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	51,508
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 18,224

Redeemable Convertible Preferred Stock - Narrative (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Apr. 30, 2019	Dec. 31, 2018	Oct. 31, 2015
Temporary Equity [Line Items]
Temporary equity, shares issued (in shares)	0		21,233,190
Temporary equity, shares outstanding (in shares)	0		21,233,190
IPO
Temporary Equity [Line Items]
Stock issued, price per share (in USD per share)	$ 20.00	$ 20.00
Preferred Stock Warrants
Temporary Equity [Line Items]
Warrants outstanding (in shares)	0		2,672,502	163,877
Preferred Stock Warrants \| IPO
Temporary Equity [Line Items]
Shares of common stock converted (in shares)	1,945,365
Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued (in shares)	0
Temporary equity, shares outstanding (in shares)	0

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Restrictions on Cash and Cash Equivalents

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows (in thousands):


	December 31,
	2019		2018
Cash and cash equivalents	$	39,181	$	24,990
Restricted cash	310		310
Total cash, cash equivalents and restricted cash	$	39,491	$	25,300

Schedule of Cash and Cash Equivalents


	December 31,
	2019		2018
Cash and cash equivalents	$	39,181	$	24,990
Restricted cash	310		310
Total cash, cash equivalents and restricted cash	$	39,491	$	25,300

Schedule of ASC 842 Impact of Adoption on Condensed Consolidated Balance Sheet

The disclosure impact of the adoption of ASC 842 on the balance sheet was as follows (in thousands):


Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

Balance sheet information as of December 31, 2019 consists of the following (in thousands):


Operating Lease:	December 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,400
Operating lease liability in accrued liabilities	$	769
Operating lease liability in other liabilities	3,700
Total operating lease liabilities	$	4,469

Net Loss Per Share Determination

Net loss per share was determined as follows (in thousands, except share and per share data):


	Year Ended December 31,
	2019		2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(52,415)	$	(37,629)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	22,956,679		960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(2.28)	$	(39.16)

Schedule of Potentially Dilutive Securities Outstanding Excluded from Diluted Weighted Average Shares Outstanding

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted average shares outstanding because such securities have an antidilutive impact due to the Company’s net loss:


	December 31,
	2019	2018
Redeemable convertible preferred stock outstanding	—	21,233,190
Redeemable convertible preferred stock warrants outstanding	—	2,672,502
Common stock options	4,310,790	4,364,377
Common stock warrants outstanding	—	7,527
Total	4,310,790	28,277,596

Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - Accounting Standards Update 2016-09 $ in Thousands	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle/treatment	$ 0
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle/treatment	$ 13

Commitments and Contingencies - Topic 840 Aggregate Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
Total	$ 5,999

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Basis of Preparation

The accompanying financial statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Adjustment to Prior Period Financial Statements

The Company has adjusted the accompanying December 31, 2018 balance sheet to increase each of the accounts receivable and accrued liabilities balances by $1,860,000 to correct for an immaterial prior year error in the classification of provisions for returns from customers.

Principles of Consolidation

Principles of ConsolidationAs of December 31, 2017, the consolidated financial statements of the Company include the accounts of Silk Road Medical, Inc. and its consolidated variable interest entity (“VIE”). Disclosure regarding the Company’s participation in the VIE is included in Note 12, “Variable Interest Entity – NeuroCo”. On December 17, 2018, the Company acquired all assets and assumed all liabilities of its VIE. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses judgment when making estimates related to provisions for accounts receivable and excess and obsolete inventories, the valuation of deferred tax assets, the reserves for sales returns, stock-based compensation, and for periods prior to the Company's IPO, the valuation of common stock and redeemable convertible preferred stock warrants. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately differ from these estimates and assumptions.

Fair Value of Financial Instruments

The Company has evaluated the estimated fair value of its financial instruments as of December 31, 2019 and 2018. The carrying amounts of certain of the Company’s financial instruments, which include cash equivalents, short-term investments, long-term investments, restricted cash, accounts receivable, accounts payable and accrued liabilities approximate their respective fair values because of the short-term nature of these instruments. Management believes that its long-term debt bears interest at the prevailing market rates for instruments with similar characteristics (Level 2 within the fair value hierarchy); accordingly, the carrying value of this instrument approximates its fair value. Prior to the Company's IPO, fair value accounting was applied to the redeemable convertible preferred stock warrant liability.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of December 31, 2019 and 2018, the Company’s cash equivalents are entirely comprised of investments in money market funds.


	December 31,
	2019		2018
Cash and cash equivalents	$	39,181	$	24,990
Restricted cash	310		310
Total cash, cash equivalents and restricted cash	$	39,491	$	25,300

Restricted cash as of December 31, 2019 and 2018 consists of a letter of credit of $310,000 representing collateral for the Company’s facility lease.

Investments

Short-term investments consist of debt securities classified as available-for-sale and have original maturities greater than 90 days, but less than one year as of the balance sheet date. Long-term investments have maturities greater than one year as of the balance sheet date. All investments are recorded at fair value based on the fair value hierarchy. Money market funds are classified within Level 1 of the fair value hierarchy, and commercial paper, corporate bonds/notes, United States Government securities, and asset-backed securities are classified within Level 2 of the fair value hierarchy. Unrealized gains and losses, deemed temporary in nature, are reported as a separate component of accumulated other comprehensive income (loss). The cost of available-for-sale investments sold is based on the specific-identification method. Realized gains and losses are included in earnings, and are derived for specific-identification method for determining the costs of investments sold. Amortization of premiums and accretion of discounts are reported as a component of interest income.

A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the investment.

Concentration of Credit Risk, and Other Risks and Uncertainties

Financial instruments that potentially subject the Company to credit risk consist of cash and cash equivalents, investments and accounts receivable to the extent of the amounts recorded on the balance sheet. Cash, cash equivalents, and investments are deposited in financial institutions which, at times, may be in excess of federally insured limits. Cash equivalents are invested in highly rated money market funds. The Company invests in a variety of financial instruments, such as, but not limited to, commercial paper, corporate bonds/notes, United States Government securities, asset-backed securities and, by policy, limits the amount of credit exposure with any one financial institution or commercial issuer. The Company has not experienced any material losses on its deposits of cash and cash equivalents or investments during the years ended December 31, 2019 and 2018.

The Company’s accounts receivable are due from a variety of health care organizations in the United States. At December 31, 2019 and 2018, no customer represented 10% or more of the Company’s accounts receivable. For the years ended December 31, 2019 and 2018, there were no customers that represented 10% or more of revenue.

The Company provides for uncollectible amounts when specific credit problems are identified. In doing so, the Company analyzes historical bad debt trends, customer credit worthiness, current economic trends and changes in customer payment patterns when evaluating the adequacy of the allowance for doubtful accounts.

The Company manufactures certain of its commercial products in-house. Certain of the Company’s product components and sub-assemblies continue to be manufactured by sole suppliers, the most significant of which is the ENROUTE stent. Disruption in component or sub-assembly supply from these manufacturers or from in-house production would have a negative impact on the Company’s financial position and results of operations.

The Company is subject to certain risks, including that its devices may not be approved or cleared for marketing by governmental authorities or be successfully marketed. There can be no assurance that the Company’s products will achieve widespread adoption in the marketplace, nor can there be any assurance that existing devices or any future devices can be developed or manufactured at an acceptable cost and with appropriate performance characteristics. The Company is also subject to risks common to companies in the medical device industry, including, but not limited to, new technological innovations, dependence upon third-party payers to provide adequate coverage and reimbursement, dependence on key personnel and suppliers, protection of proprietary technology, product liability claims, and compliance with government regulations.

Existing or future devices developed by the Company may require approvals or clearances from the FDA or international regulatory agencies. In addition, in order to continue the Company’s operations, compliance with various federal and state laws is required. If the Company were denied or delayed in receiving such approvals or clearances, it may be necessary to adjust operations to align with the Company’s currently approved portfolio. If clearance for the products in the current portfolio were withdrawn by the FDA, this would have a material adverse impact on the Company.

Accounts Receivable

Trade accounts receivable are recorded at the invoiced amount and do not bear interest. The Company estimates allowances for doubtful accounts. Specifically, the Company makes estimates on the collectability of customer accounts based primarily on analysis of historical trends and experience and changes in customers’ financial condition. The Company uses its judgment, based on the best available facts and circumstances, and records an allowance against amounts due to reduce the receivable to the amount that is expected to be collected. These specific allowances are reevaluated and adjusted as additional information is received that impacts the amount reserved. During the years ended December 31, 2019 and 2018, the Company did not experience any material credit-related losses.

Inventories

Inventories are valued at the lower of cost to purchase or manufacture the inventory or net realizable value. Cost is determined using the first-in, first-out method for all inventories. Net realizable value is determined as the estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. The Company regularly reviews inventory quantities in consideration of actual loss experiences, projected future demand, and remaining shelf life prior to sale to record a provision for excess and obsolete inventory when appropriate. The Company’s policy is to write down inventory that has become obsolete, inventory that has a cost basis in excess of its expected lower of cost or net realizable value, and inventory in excess of expected requirements. The estimate of excess quantities is judgmental and primarily dependent on the Company’s estimates of future demand for a particular product. If the estimate of future demand is too high, the Company may have to increase the reserve for excess inventory for that product and record a charge to the cost of goods sold.

Property and Equipment

Property and equipment are recorded at cost less accumulated depreciation or amortization. Repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the assets, typically three to five years. Leasehold

improvements are amortized using the straight-line method over the shorter of the lease term or estimated useful economic life of the asset. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized.

Deferred Public Offering Costs

Deferred Public Offering CostsSpecific incremental legal, accounting and other fees and costs directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. As of December 31, 2018, there were $950,000 of offering costs primarily consisting of legal and accounting fees that were capitalized in other non-current assets on the balance sheet of which $233,000 had been paid. No deferred offering costs were capitalized as of December 31, 2019.

Impairment of Long-Lived Assets

Impairment of Long-Lived AssetsThe Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If indicators of impairment exist, an impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the assets and their eventual disposition are less than their carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of the long-lived assets exceeds their fair value.

Leases

The Company adopted Accounting Standards Codification (“ASC”) 842, "Leases," on January 1, 2019 and used the modified retrospective method for all leases not substantially completed as of the date of adoption and the package of practical expedients available in the standard. As a result of adopting ASC 842, the Company recorded an operating lease right-of-use ("ROU") asset of $3,982,000 included within other non-current assets and operating lease liabilities of $5,190,000 included within accrued liabilities and other liabilities on the balance sheet related to its facility lease, based on the present value of the future lease payments on the date of adoption. The operating lease right-of-use asset also includes adjustments for prepayments and excludes lease incentives. The adoption did not have an impact on prior periods or on the Company's statements of operations and comprehensive loss.

The disclosure impact of the adoption of ASC 842 on the balance sheet was as follows (in thousands):


Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

The Company considers if an arrangement is a lease at inception if it obtains the right to control the use of an identified asset under a leasing arrangement with an initial term greater than twelve months. The Company determines whether a contract conveys the right to control the use of an identified asset for a period of time if the contract contains both the right to obtain substantially all of the economic benefits from the use of the identified asset and the right to direct the use of the identified asset. The Company also evaluates the nature of each lease to determine whether it is an operating or financing lease and recognizes the right-of-use asset and lease liabilities based on the present value of future minimum lease

payments over the expected lease term. The Company’s leases do not generally contain an implicit interest rate and therefore the Company uses the incremental borrowing rate it would expect to pay to borrow on a similar collateralized basis over a similar term in order to determine the present value of its lease payments. The Company’s considers renewal options in the determination of the lease term if the option to renew is reasonably certain. Variable lease costs represent payments that are dependent on usage, a rate or index. Variable lease costs, which consists primarily of taxes, insurance and common area maintenance costs, are expensed as incurred, as the Company has elected to account separately for contracts that contain lease and non-lease components, consistent with its historical practice. The Company does not have any finance leases.

Redeemable Convertible Preferred Stock Warrant Liability

Prior to its IPO, the Company accounted for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability were initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement was recognized as a component of other income (expense), net in the statements of operations and comprehensive loss. The Company recorded adjustments to the estimated fair value of the redeemable convertible preferred stock warrants until they were exercised. Upon their exercise, the final fair value of the warrant liability was reclassified to stockholders’ equity (deficit). Subsequent to its IPO, the Company no longer recorded any related periodic fair value adjustments.

Redeemable Convertible Preferred Stock

Prior to its IPO, the Company recorded its redeemable convertible preferred stock at fair value on the dates of issuance, net of issuance costs, and classified the redeemable convertible preferred stock outside of stockholders’ equity (deficit) on the balance sheet as events triggering the liquidation preferences were not solely within the Company’s control. Upon the closing of the Company's IPO, all shares of convertible preferred stock then outstanding converted into an aggregate of 23,178,555 shares of common stock resulting in the reclassification of $144,140,000 from outside of stockholders’ equity (deficit) to additional paid-in capital.

Revenue Recognition and Cost of Goods Sold

Revenue Recognition

On January 1, 2018, the Company adopted ASC Topic 606, “Revenue from Contracts with Customers,” using the modified retrospective method applied to contracts which were not completed as of that date. Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps:

(i)identify the contract(s) with a customer;

(ii)identify the performance obligations in the contract;

(iii)determine the transaction price;

(iv)allocate the transaction price to the performance obligations in the contract; and

(v)recognize revenue when (or as) the entity satisfies a performance obligation.

Under ASC 606, assuming all other revenue recognition criteria have been met, the Company will recognize revenue earlier for arrangements where the Company has satisfied its performance obligations

but have not issued invoices. As of December 31, 2019 and 2018, the Company recorded $102,000 and $128,000, respectively, of unbilled receivables, which are included in accounts receivable, net on the balance sheet, as the Company has an unconditional right to payment as of the end of the applicable period.

The Company’s revenue is generated from the sale of its products to hospitals and medical centers in the United States through direct sales representatives. Revenue is recognized when obligations under the terms of a contract with customers are satisfied, which occurs with the transfer of control of the Company’s products to its customers, either upon shipment of the product or delivery of the product to the customer under the Company’s standard terms and conditions. The Company’s products are readily available for usage as soon as the customer possesses it. Upon receipt, the customer controls the economic benefits of the product, has significant risks and rewards, and the legal title. The Company has present right to payment; therefore, the transfer of control is deemed to happen at a point in time. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring the goods.

For sales where the Company’s sales representative hand delivers product directly to the hospital or medical center from the sales representative’s trunk stock inventory, the Company recognizes revenue upon delivery, which represents the point in time when control transfers to the customer. Upon delivery there are legally-enforceable rights and obligations between the parties which can be identified, commercial substance exists and collectibility is probable. For sales which are sent directly from the Company to hospitals and medical centers, the transfer of control occurs at the time of shipment or delivery of the product. There are no further performance obligations by the Company or the sales representative to the customer after delivery under either method of sale. As allowed under the practical expedient, the Company does not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which it recognizes revenue at the amount to which it has the right to invoice for services performed.

The Company is entitled to the total consideration for the products ordered by customers as product pricing is fixed according to the terms of customer contracts and payment terms are short. Payment terms fall within the one-year guidance for the practical expedient which allows the Company to forgo adjustment of the promised amount of consideration for the effects of a significant financing component. The Company excludes taxes assessed by governmental authorities on revenue-producing transactions from the measurement of the transaction price.

Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of revenue in the statements of operations and comprehensive loss.

The Company accepts product returns at its discretion or if the product is defective as manufactured. The Company establishes estimated provisions for returns based on historical experience and considers other factors that it believes could significantly impact its expected returns, which provisions are classified within accrued liabilities on our balance sheet. The Company elected to expense shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where the Company bills shipping and handling costs to customers, it will classify the amounts billed as a component of revenue.

Cost of Goods Sold

The Company manufactures certain of its portfolio of TCAR products at its facility and purchases other products from third party manufacturers. Cost of goods sold consists primarily of costs related to

materials, components and subassemblies, manufacturing overhead costs, direct labor, reserves for excess, obsolete and non-sellable inventories as well as distribution-related expenses. A significant portion of the Company’s cost of goods sold currently consists of manufacturing overhead costs. These overhead costs include the cost of quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs and royalties.

Research and Development and Clinical Trials

Research and Development

The Company expenses research and development costs as incurred. Research and development expenses consist primarily of engineering, product development, clinical studies to develop and support the Company’s products, regulatory expenses, medical affairs and other costs associated with products and technologies that are in development. Research and development expenses include employee compensation, including stock-based compensation, supplies, consulting, prototyping, testing, materials, travel expenses, depreciation and an allocation of facility overhead expenses. Additionally, research and development expenses include costs associated with our clinical studies including clinical trial design, clinical site reimbursement, data management, travel expenses and the cost of products used for clinical trials and internal and external costs associated with the Company’s regulatory compliance and quality assurance functions, including the costs of outside consultants and contractors that assist in the process of submitting and maintaining regulatory filings, and overhead costs.

Clinical Trials

The Company accrues and expenses costs for its clinical trial activities performed by third parties, including clinical research organizations and other service providers, based upon estimates of the work completed over the life of the individual study in accordance with associated agreements. The Company determines these accruals through discussion with internal personnel and outside service providers as to progress or stage of completion of trials or services pursuant to contracts with clinical research organizations and other service providers and the agreed-upon fee to be paid for such services.

Advertising Costs

Advertising CostsThe Company expenses advertising costs as incurred. Advertising costs include design and production costs, including website development, physician and patient testimonial videos, written media campaigns, and other items.

Foreign Currency

The Company records net gains and losses resulting from foreign exchange transactions as a component of foreign currency exchange gains or losses in other income (expense), net. The Company had no material foreign currency exchange gains or losses during the years ended December 31, 2019 and 2018.

Stock-Based Compensation

Stock–Based Compensation

The Company accounts for stock-based compensation in accordance with Financial Accounting Standards Board (“FASB”) ASC 718, “Compensation-Stock Compensation.” ASC 718 requires the recognition of compensation expense, using a fair-value based method, for costs related to all share-based payments including stock options. ASC 718 requires companies to estimate the fair value of all share-based payment option awards on the date of grant using an option pricing model. The fair value of stock options is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period), on a

straight-line basis. For performance-based stock options, the Company will assess the probability of performance conditions being achieved in each reporting period. The amount of stock-based compensation expense recognized in any one period related to performance-based stock options can vary based on the achievement or anticipated achievement of the performance conditions.

In March 2016, the FASB issued Accounting Standards Update ("ASU") No. 2016-09, “Stock Compensation (Topic 718): Improvements to Employee Shared-Based Payment Accounting.” Under ASU 2016-09, entities are permitted to make an accounting policy election to either estimate forfeitures on share-based payment awards, as previously required, or to recognize forfeitures as they occur. The Company made an accounting policy election to account for forfeitures as they occur. This change has been applied on a modified retrospective basis, resulting in a cumulative-effect adjustment to increase the beginning accumulated deficit by $13,000 as of January 1, 2018, the date of adoption.

Income Taxes

The Company accounts for income taxes under the liability method, whereby deferred tax assets and liabilities are determined based on the difference between the financial statements and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized.

The Company also follows the provisions of ASC 740-10, “Accounting for Uncertainty in Income Taxes.” ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the financial statements. It is the Company’s policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.

Comprehensive Loss

Comprehensive LossComprehensive loss consists of net loss and changes in unrealized gains and losses on investments classified as available-for-sale. For the year ended December 31, 2019, the Company’s unrealized gains and losses on available-for-sale investments represent the only component of other comprehensive loss that are excluded from the reported net loss and that are presented in the statements of operations and comprehensive income. Accumulated other comprehensive loss is presented in the accompanying balance sheets as a component of stockholders' equity (deficit). For the year ended December 31, 2018, there was no difference between the Company's comprehensive loss and its net loss.

Net Loss per Share Attributable to Common Stockholders and Unaudited Pro Forma Net Loss per Share Attributable to Common Stockholders

Net Loss per Share Attributable to Common Stockholders

Basic net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding during the period, without consideration for potential dilutive common shares. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and warrants, and common stock options are considered to be potentially dilutive securities. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive.

The Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The shares of the Company’s redeemable convertible preferred

stock participate in any dividends declared by the Company and are therefore considered to be participating securities.

Net loss per share was determined as follows (in thousands, except share and per share data):


	Year Ended December 31,
	2019		2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(52,415)	$	(37,629)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	22,956,679		960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(2.28)	$	(39.16)


	December 31,
	2019	2018
Redeemable convertible preferred stock outstanding	—	21,233,190
Redeemable convertible preferred stock warrants outstanding	—	2,672,502
Common stock options	4,310,790	4,364,377
Common stock warrants outstanding	—	7,527
Total	4,310,790	28,277,596

Segment and Geographical Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment. All of the Company’s revenue was in the United States for the years ended December 31, 2019 and 2018, based on the shipping location of the external customer.

Recent Accounting Pronouncements

Recently Adopted Accounting Standards

In February 2016, the FASB issued ASU No. 2016-02, "Leases," that supersedes ASC 840, "Leases." Subsequently, the FASB issued several updates to ASU No. 2016-02, codified in ASC Topic 842 (“ASC 842”). The Company adopted ASC 842 on January 1, 2019 using the modified retrospective method for all leases not substantially completed as of the date of adoption. The Company elected to apply the package of practical expedients, which allowed the Company to not reassess: (i) whether expired or existing contracts contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs for any existing lease.

Recently Issued Accounting Standards

In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Statements. This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement, which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. ASU 2018-13 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, Cloud Computing Arrangements, which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. ASU 2018-15 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements and related disclosures.

In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance related to intraperiod tax allocation, interim period accounting for enacted changes in tax law, and the year-to-date loss limitation in interim period tax accounting. ASU 2019-12 also amends other aspects of the guidance to reduce complexity in certain areas. ASU 2019-12 will become effective for the Company on January 1, 2021. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its financial statements and related disclosures.

Fair Value Measurements - Narrative (Details)	Dec. 31, 2018 $ / shares
Common Stock
Class of Stock [Line Items]
Fair value of stock (in USD per share)	$ 11.29

Balance Sheet Components - Available-for-Sale Investments in Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	$ 29,195
Unrealized Loss, Less than 12 months	(7)
Corporate bonds/notes
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	10,128
Unrealized Loss, Less than 12 months	(4)
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 months	19,067
Unrealized Loss, Less than 12 months	$ (3)

Balance Sheet Components - Schedule of Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Raw materials	$ 1,203	$ 1,054
Finished products	9,119	4,690
Total inventory	$ 10,322	$ 5,744

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Earnings per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,310,790	28,277,596
Redeemable convertible preferred stock outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	0	21,233,190
Redeemable convertible preferred stock warrants outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	0	2,672,502
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,310,790	4,364,377
Common stock warrants outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	0	7,527

Long-term Debt - Series C Narrative (Details) - $ / shares	1 Months Ended
	Jul. 31, 2017	Oct. 31, 2015	Dec. 31, 2019	Dec. 31, 2018
Preferred Stock Warrants
Class of Stock [Line Items]
Warrants outstanding (in shares)		163,877	0	2,672,502
Series C
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)	163,877	327,759
Stock issued, price per share (in USD per share)	$ 6.11	$ 6.11

Stock Option Plans - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)		848,023	629,716
Weighted average grant date fair value (in USD per share)		$ 10.17	$ 2.93
Fair value of options vested (in USD per share)		$ 2,221,000	$ 569,000
Aggregate intrinsic value of options exercised		$ 24,867	$ 787
Compensation expense for share-based awards		$ 2,977	$ 911
Common stock reserved for future issuance (in shares)		6,237,832	28,499,408
Compensation expensed not yet recognized		$ 8,539
Compensation expensed not yet recognized, period for recognition		3 years 14 days
ISO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		100.00%
NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		85.00%
ISO and NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option term		10 years
Option vesting term		4 years
ISO and NSO \| 10% Stockholders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares		110.00%
2019 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares of common stock	1.00%
Percent of purchase of price of common stock	85.00%
Common stock reserved for future issuance (in shares)	434,000	372,352,000
Number of additional shares allowable under the plan (in shares)	1,200,000
Compensation expensed not yet recognized		$ 219
Compensation expensed not yet recognized, period for recognition		4 months 20 days
Issuance of common stock under employee stock purchase plan (in shares)		61,648,000
2019 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)	2,317,000	2,366,251
Percent of outstanding shares of common stock	4.00%
2019 Stock Option Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares allowable under the plan (in shares)	3,000,000

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

The components of income before taxes are as follows (in thousands):


	Year Ended December 31,
	2019		2018
United States	$	(52,415)	$	(37,630)
International	—		—
	$	(52,415)	$	(37,630)

A reconciliation of the statutory U.S. federal rate to the Company’s effective tax rate is as follows (in thousands):


	Year Ended December 31,
	2019		2018
Tax at federal statutory rate	$	(11,007)	$	(7,902)
State taxes, net of federal benefit	(2,270)		(1,582)
Permanent differences	(4,731)		289
Loss on Series C warrant liability	5,330		2,500
Change in valuation allowance	12,797		6,197
General business credits	(319)		136
Other	208		376
Provision for income taxes	$	8	$	14

The Company's provision for income taxes are included within other income (expense) on the statements of operations and comprehensive loss.

Significant components of the Company’s net deferred tax assets as of December 31, 2019 and 2018 consist of the following (in thousands):


	December 31,
	2019		2018
Deferred tax assets:
Net operating loss carryforwards	$	44,574	$	33,815
Research and development credits	5,287		4,944
Capitalized start-up costs/Intangibles	11		16
Accruals and reserves	2,227		1,169
Property and equipment	188		82
Stock-based compensation	568		274
Operating lease liability	1,133		—
Total deferred tax assets	53,988		40,300
Less: Valuation allowance	(53,126)		(40,300)
Deferred tax liabilities:
Operating lease asset	(862)		—
Total deferred tax liabilities	(862)		—
Net deferred tax assets	$	—	$	—

In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management believes it is more likely than not that the deferred tax assets will not be realized; accordingly, a valuation allowance has been established on U.S. net deferred tax assets. The valuation allowance increased $12,826,000 during the year ended December 31, 2019 and increased by $6,197,000 during the year ended December 31, 2018.

As of December 31, 2019, the Company had net operating loss carryforwards of approximately $167,948,000 and $148,380,000 for federal and state income tax purposes, respectively. The federal and state net operating loss carryforwards begin to expire in 2027 and 2028, respectively. Federal NOL carryforwards generated in tax years beginning in 2018 are not subject to expiration. Federal NOLs that arose on or after January 1, 2018 can be carried forward indefinitely against future income, but can only be used to offset a maximum of 80% of the Company's federal taxable income in any year.

The federal and state net operating loss carryforwards may be subject to significant limitations under Section 382 and Section 383 of the Internal Revenue Code and similar provisions under state law. Federal tax legislation enacted in December 2017, commonly known as the Tax Cuts and Jobs Act, contains provisions that limit the federal net operating loss carryforwards that may be used in any given year in the event of special occurrences, including significant ownership changes. A Section 382 “ownership change” generally occurs if one or more stockholders or groups of stockholders, who own at least 5% of the Company’s stock, increase their ownership by more than 50 percentage points over their lowest ownership percentage within a rolling three-year period. The Company may have previously experienced, and may in the future experience, one or more Section 382 “ownership changes,” including in connection with the Company’s initial public offering. If so, the Company may lose some or all of the tax benefits of its NOLs and tax credits. The extent of such limitations for prior years, if any, has not yet been formally determined.

At December 31, 2019, the Company had $4,320,000 and $2,860,000 of federal and state research and development credit carryforwards, respectively. If not utilized, the federal credits will expire beginning in 2027. The California Research and Development credits can be carried forward indefinitely.

As of December 31, 2019, the Company had $1,436,000 of unrecognized tax benefits. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized would increase or decrease within twelve months of the year ended December 31, 2019. If recognized, $0 would affect the effective tax rate.

The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. There was no such expense recorded during the years ended December 31, 2019 and 2018.

A reconciliation of the unrecognized tax benefits from January 1, 2018 to December 31, 2019 is as follows (in thousands):


	December 31,
	2019		2018
Balance at the beginning of year	$	1,348	$	615
Increases related to current years’ tax positions	88		118
Increases/(decreases) related to prior years’ tax positions	—		615
Balance at end of year	$	1,436	$	1,348

The Company currently has no federal or state tax examinations in progress nor has it had any federal or state tax examinations since its inception. As a result of the Company’s net operating loss carryforwards, all of its tax years are subject to federal and state tax examination.

Acquisition of Variable Interest Entity - NeuroCo (Details) - USD ($)		12 Months Ended
	Dec. 17, 2018	Dec. 31, 2019	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Value of common stock issued		$ 109,119,000
Carrying amount of the non-controlling interest	$ 0
NeuroCo, Inc
Business Acquisition [Line Items]
Equity interest acquired	100.00%
Shares issued as consideration (in shares)	33,462
Consideration given, amount of notes payable forgiven	$ 1,600,000
NeuroCo, Inc
Business Acquisition [Line Items]
Promissory note amount				$ 498,000
Notes receivable, interest rate				2.74%
Notes receivable, term				10 years
Value of common stock issued			$ 154,000

Label	Element	Value
Accounting Standards Update 2016-09 [Member] \| Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 13,000
Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	87,000
Accounting Standards Update 2014-09 [Member] \| Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 87,000

Summary of Significant Accounting Policies - Segment and Geographical Information (Details)	12 Months Ended
	Dec. 31, 2019 segment
Accounting Policies [Abstract]
Number of reportable segments	1
Number of operating segments	1

Long-term Debt

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Long-term Debt

In October 2015, the Company entered into a term loan agreement with Capital Resource Group ("CRG"). The term loan agreement provides for up to $30,000,000 in term loans split into two tranches as follows: (i) the Tranche A Loans provided for $20,000,000 in term loans, and (ii) the Tranche B Loans provided for up to $10,000,000 in term loans. The Company drew down the Tranche A Loans on October 13, 2015. The Tranche B Loans were available to be drawn prior to March 29, 2017. In January 2017, the term loan agreement was amended to extend the commitment period of the Tranche B Loans to April 28, 2017. In April 2017, the Company drew down $5,000,000 of the available Tranche B Loans.

In September 2018, the Company entered into Amendment No. 5 to the term loan agreement with CRG. Under the amended terms of the amended loan agreement the maturity date was extended to December 31, 2022 and the repayment schedule of the existing term loans were changed to interest only so that the outstanding principal amount of the term loans will be payable in a single installment at maturity. The related fixed interest rate was changed to equal 10.75% per annum, due and payable quarterly in arrears. At the election of the Company, 2.75% of the interest due and payable may be “paid in kind” and added to the then outstanding principal and 8.0% of the interest due and payable paid in cash. All unpaid principal, and accrued and unpaid interest, is due and payable in full on December 31, 2022. The amended term loan agreement also provided for additional term loans in an aggregate principal amount of up to $25,000,000 and allowed for the conversion into shares of common stock, at the Company’s option, of up to 25% of the outstanding loans under the term loan agreement in connection with an initial public offering of the Company’s common stock which results in market capitalization of at least $250,000,000. In September 2018, the Company drew down an additional $15,000,000 under the term loan agreement with CRG.

In June 2019, the Company entered into Amendment No. 7 to the term loan agreement with CRG to reflect flexibility with respect to permitted cash investments.

The Company may voluntarily prepay the borrowings in full. The Tranche A borrowing required a payment, on the borrowing date, of a financing fee equal to 1.75% of the borrowed loan principal, which is recorded as a discount to the debt. In addition, a facility fee equal to 5.0% of the amounts borrowed plus any “paid in kind” is payable at the end of the term or when the borrowings are repaid in full. A long-term

liability is being accreted using the effective interest method for the facility fee over the term of the loan agreement. The borrowings are collateralized by a security interest in substantially all of the Company’s assets.

The Company is subject to financial covenants related to liquidity and minimum trailing revenue targets that begin in December 31, 2016 and are tested on an annual basis. The liquidity covenant requires the Company to maintain an amount which shall exceed the greater of (i) $3,000,000 and (ii) the minimum cash balance, if any, required of the Company by a creditor to the extent the Company has incurred permitted priority debt. The Company had to achieve minimum net revenue of $1,000,000 in 2016, $5,000,000 in 2017, $15,000,000 in 2018, and must achieve minimum net revenue of $30,000,000 in 2019 and $40,000,000 in 2020. The liquidity financial covenant has a 90-day equity cure period following end of the calendar year to issue additional shares of equity interests in exchange for cash, or to borrow permitted cure debt. In addition, the term loan agreement prohibits the payment of cash dividends on the Company’s capital stock and also places restrictions on mergers, sales of assets, investments, incurrence of liens, incurrence of indebtedness and transactions with affiliates. CRG may accelerate the payment terms of the term loan agreement upon the occurrence of certain events of default set forth therein, which include the failure of the Company to make timely payments of amounts due under the term loan agreement, the failure of the Company to adhere to the covenants set forth in the term loan agreement, the insolvency of the Company or upon the occurrence of a material adverse change. As of December 31, 2019, the Company was in compliance with all applicable financial covenants. As of December 31, 2019, management does not believe that it is probable that the above clauses will be triggered within the next twelve months, therefore, the debt is classified as long-term on the balance sheet.

Future maturities under the term loan agreement as of December 31, 2019 are as follows (in thousands):


Year Ending December 31:	Amount
2020	$	4,454
2021	4,442
2022	48,256
	57,152
Add: Accretion of closing fees	1,205
	58,357
Less: Amount representing interest	(13,339)
Less: Amount representing debt discount and debt issuance costs	(139)
Present value of minimum payments	$	44,879

In October 2015, CRG purchased 327,759 shares of the Company’s Series C redeemable convertible preferred stock at $6.11 per share. In addition, CRG received warrants to purchase 163,877 shares of the Company’s Series C redeemable convertible preferred stock at an exercise price of $6.11 per share. Upon the closing of the IPO, the warrants were net exercised, based on the IPO price of $20.00 per share, into shares of common stock.

In July 2017, CRG purchased 163,877 shares of the Company’s Series C convertible preferred stock at $6.11 per share.

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Increase (decrease) in deferred tax asset valuation allowance	$ 12,826,000	$ 6,197,000
Percent maximum of federal taxable income to offset with net operating losses	80.00%
Unrecognized tax benefits	$ 1,436,000	1,348,000	$ 615,000
Unrecognized tax benefits that would impact effective tax rate	0
Unrecognized tax benefits, income tax penalties and interest expense	0	$ 0
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	167,948,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	148,380,000
Research Tax Credit Carryforward \| Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Research and development tax credits	4,320,000
Research Tax Credit Carryforward \| State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Research and development tax credits	$ 2,860,000

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events 2019 Equity Incentive Plan In January 2020, the number of shares of common stock authorized for issuance under the 2019 Plan was automatically increased by 1,250,210 shares, which was ratified by the Company's Board of Directors in February 2020. In February 2020, the Company’s Board of Directors approved the grant of options to purchase 92,885 shares of common stock under the 2019 Plan with an exercise price of $47.20 per share. 2019 Employee Stock Purchase Plan In January 2020, the number of shares of common stock authorized for issuance under the 2019 ESPP was automatically increased by 312,552 shares, which was ratified by the Company's Board of Directors in February 2020.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Lease and Rights of Use

The Company’s operating lease obligation consists of leased office, laboratory, and manufacturing space under a non-cancellable operating lease that expires in October 2024. The lease agreement includes a renewal provision allowing the Company to extend this lease for an additional period of five years. Operating lease costs were $870,000 for the year ended December 31, 2019. Cash paid for amounts included in the measurement of operating lease liabilities was $721,000 for the year ended December 31, 2019. As of December 31, 2019, the weighted average discount rate was approximately 6.50% and the weighted average remaining lease term was 4.83 years. Balance sheet information as of December 31, 2019 consists of the following (in thousands):


Operating Lease:	December 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,400
Operating lease liability in accrued liabilities	$	769
Operating lease liability in other liabilities	3,700
Total operating lease liabilities	$	4,469

The following table summarizes the Company's operating lease maturities as of December 31, 2019 (in thousands):


Year Ending December 31:	Amount
2020	$	1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,230
Less: imputed interest	(761)
Present value of lease liabilities	$	4,469

The aggregate future minimum lease payments under ASC 840 as of December 31, 2018 were as follows (in thousands):


Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
	$	5,999

Purchase Obligations

Purchase obligations consist of agreements to purchase goods and services entered into in the ordinary course of business. As of December 31, 2019, the Company had non-cancellable purchase obligations to suppliers of $7,097,000.

Indemnification

In the normal course of business, the Company enters into contracts and agreements with suppliers and other parties that contain a variety of representations and warranties and may provide for indemnification of the counterparty. The Company’s exposure under these agreements is unknown because it involves claims that may be made against it in the future but have not yet been made. To date, the Company has not been subject to any claims or been required to defend any action related to its indemnification obligations.

The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director liability insurance. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations as of December 31, 2019.

Contingencies

The Company is not involved in any pending legal proceedings that it believes could have a material adverse effect on its financial condition, results of operations or cash flows. From time to time, the Company may pursue litigation to assert its legal right and such litigation may be costly and divert the efforts and attention of its management and technical personnel which could adversely affect its business. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual at December 31, 2019 and 2018.

Legal Matters

In February 2019, a former employee, through counsel, advised the Company that he had filed a charge of discrimination against the Company with the California Department of Fair Employment & Housing, or

DFEH. The former employee’s complaint alleged sexual harassment and retaliation in violation of the California Department of Fair Employment & Housing Act. The complaint did not allege specific damages. The Company and the former employee participated in mediation on July 30, 2019 and reached a settlement that required the Company to pay an amount that was not material to its financial statements, which amount was fully paid as of December 31, 2019.

Stock Option Plans - Stock-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,977	$ 911
Cost of goods sold
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	179	51
Research and development expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	426	256
Selling, general and administrative expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,372	$ 604

Balance Sheet Components - Fair Value of Cash Equivalents, Short-Term and Long-Term Investments (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Amounts maturing within one year	$ 85,871
Amounts maturing after one year through two years	18,224
Total	$ 104,095

Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset	$ 3,400
Operating lease liabilities	$ 4,469
Adjustments Due to ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset		$ 3,982
Operating lease liabilities		$ 5,190

Formation and Business of the Company (Details) $ / shares in Units, $ in Thousands		1 Months Ended		12 Months Ended
	Mar. 13, 2019	Aug. 31, 2019 $ / shares shares	Apr. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Subsequent Event [Line Items]
Accumulated deficit				$ 191,526	$ 139,111
Cash and cash equivalents				39,181	24,990
Available-for-sale investments				69,732
Reverse stock split	0.3704
Payments of deferred offering costs				2,481	233
Accounts receivable				$ 8,601	$ 6,382
IPO
Subsequent Event [Line Items]
Issued and sold common stock (in shares) \| shares			6,000,000
Net proceeds			$ 109,119
Stock issued, price per share (in USD per share) \| $ / shares			$ 20.00	$ 20.00
IPO - Underwriting Discounts And Commissions
Subsequent Event [Line Items]
Payments of deferred offering costs			$ 8,400
IPO - Other Expenses
Subsequent Event [Line Items]
Payments of deferred offering costs			$ 2,481
Second Public Offering
Subsequent Event [Line Items]
Issued and sold common stock (in shares) \| shares		4,200,000
Stock issued, price per share (in USD per share) \| $ / shares		$ 39.50
Over-Allotment Option
Subsequent Event [Line Items]
Issued and sold common stock (in shares) \| shares		630,000

401(k) Plan (Details) - USD ($)		12 Months Ended
401(k) Plan (Details) - USD ($)	Jan. 01, 2020	Dec. 31, 2019
Subsequent Event [Line Items]
Percentage employee contribution		90.00%
Employer discretionary contribution		$ 0
Subsequent Event
Subsequent Event [Line Items]
Employer contribution match percentage	50.00%
Percent of matching contribution of the employee's pay	5.00%

Summary of Significant Accounting Policies - Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	8 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019
Class of Stock [Line Items]
Conversion of preferred stock to common stock upon IPO	$ 144,140	$ 144,140
Common Stock
Class of Stock [Line Items]
Conversion of preferred stock to common stock upon IPO (in shares)	23,178,555

Cover Page - USD ($) $ in Millions	12 Months Ended
Cover Page - USD ($) $ in Millions	Dec. 31, 2019	Feb. 28, 2020
Cover page.
Entity Registrant Name	SILK ROAD MEDICAL, INC.
Entity Central Index Key	0001397702
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2019
Entity File Number	001-38847
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-8777622
Entity Address, Address Line One	1213 Innsbruck Dr.
Entity Address, City or Town	Sunnyvale
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94089
City Area Code	408
Local Phone Number	720-9002
Title of 12(b) Security	Common Stock
Trading Symbol	SILK
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Public Float		$ 818.0
Entity Common Stock, Shares Outstanding		31,353,906
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY

Stock Option Plans (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based Compensation, Stock Options, Activity

Activity under the Company’s 2007 Stock Option Plan, NeuroCo 2015 Equity Incentive Plan and 2019 Plan is set forth below:


		Options Outstanding
	Shares Available for Grant	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)





Balances, December 31, 2017	328,290	4,308,890	$	3.09	7.81	$	5,073
Authorized	223,664
Options granted	(629,716)	629,716	$	6.51
Options exercised	—	(438,578)	$	1.50
Options cancelled	135,651	(135,651)	$	1.56
Balances, December 31, 2018	57,889	4,364,377	$	3.79	7.36	$	33,132
Authorized	2,317,000
Options granted	(848,023)	848,023	$	22.77
Options exercised	—	(873,786)	$	1.77
Options cancelled	27,824	(27,824)	$	8.71
Balances December 31, 2019	1,554,690	4,310,790	$	7.91	7.27	$	140,234
Vested and exercisable at December 31, 2019		2,514,891	$	4.27	6.38	$	90,854
Vested and expected to vest at December 31, 2019		4,310,790	$	7.91	7.27	$	140,234

Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price

The following table summarizes information about stock options outstanding and vested as of December 31, 2019:


	Options Outstanding							Options Vested
Exercise Price	Options Outstanding	Weighted Average Remaining Contractual Term (in Years)	Weighted Average Exercise Price		Number Exercisable	Weighted Average Exercise Price
$1.35 - $3.16	2,072,765	5.86	$	1.84	1,770,060	$	1.71
$4.73 - $8.27	919,355	8.23	$	5.81	383,077	$	5.72
$11.29 - $20.00	1,192,804	8.73	$	16.63	352,383	$	14.46
$36.47 - $48.53	125,866	9.69	$	40.50	9,371	$	45.57
	4,310,790	7.27	$	7.91	2,514,891	$	4.27

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The fair value of employee and nonemployee stock options was estimated using the following assumptions for the years ended December 31, 2019 and 2018:


	Year Ended December 31,
	2019	2018
Expected term (in years)	5.00 - 6.25	5.00 - 6.25
Expected volatility	42.4% - 42.9%	38.0% - 38.8%
Risk-free interest rate	1.47% - 2.54%	2.68% - 2.98%
Dividend yield	—%	—%

The fair value of the shares to be issued under the Company’s 2019 ESPP was estimated using the Black-Scholes valuation model with the following assumptions for the year ended December 31, 2019:


						Year Ended December 31,
						2019
Expected term (in years)						0.50 - 0.63
Expected volatility						44.4% - 47.8%
Risk-free interest rate						1.58% - 2.45%
Dividend yield						—%

Stock-Based Compensation Expense Relating to Stock Options to Employees and Nonemployees

Total stock-based compensation expense relating to the Company’s stock options and 2019 ESPP during the years ended December 31, 2019 and 2018, is as follows (in thousands):


	Year Ended December 31,
	2019		2018
Cost of goods sold	$	179	$	51
Research and development expenses	426		256
Selling, general and administrative expenses	2,372		604
	$	2,977	$	911

401(k) Plan	12 Months Ended
401(k) Plan	Dec. 31, 2019
Retirement Benefits [Abstract]
401(k) Plan	401(k) PlanThe Company has a qualified retirement plan under section 401(k) of the Internal Revenue Code (“IRC”) under which participants may contribute up to 90% of their eligible compensation, subject to maximum deferral limits specified by the IRC. The Company may make a discretionary matching contribution to the 401(k) plan and may make a discretionary employer contribution to each eligible employee each year. Through December 31, 2019, the Company has made no contributions to the 401(k) plan. Beginning in January 2020, the Company started matching employees' contributions to the 401(k) plan at 50% of the first 5% of compensation deferred to the 401(k) plan.

Income Taxes - Components of Income Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (52,415)	$ (37,630)
International	0	0
Total	$ (52,415)	$ (37,630)

Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Shares outstanding (in shares)	31,255,267	1,135,310
Common stock, shares issued (in shares)	31,255,267	1,135,310
Common stock, shares authorized (in shares)	100,000,000	29,879,220
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Preferred shares authorized (in shares)	5,000,000	0
Preferred shares (in USD per share)	$ 0.001	$ 0.001
Liquidation preference	$ 0	$ 121,144,000
Temporary equity, shares outstanding (in shares)	0	21,233,190
Temporary equity, shares issued (in shares)	0	21,233,190
Temporary equity, shares authorized (in shares)	0	24,069,615
Temporary equity, par value (in USD per share)	$ 0.001	$ 0.001
Allowances for accounts receivables	$ 45,000	$ 22,000

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net loss	$ (52,415)	$ (37,630)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	712	517
Stock-based compensation expense	2,977	911
Change in fair value of redeemable convertible preferred stock warrant liability	21,030	11,906
Accretion of discount on investments	(309)	0
Amortization of debt discount and debt issuance costs	46	68
Amortization of right-of-use asset	582
Non-cash interest expense	672	1,555
Loss on disposal of property and equipment	0	159
Provision for doubtful accounts receivable	23	(123)
Provision for excess and obsolete inventories	118	23
Changes in assets and liabilities
Accounts receivable	(2,241)	(446)
Inventories	(4,696)	(2,565)
Prepaid expenses and other current assets	(1,471)	(1,128)
Other assets	552	(62)
Accounts payable	615	(309)
Accrued liabilities	4,964	5,023
Other liabilities	(769)	406
Net cash used in operating activities	(29,610)	(21,695)
Cash flows from investing activities
Purchase of property and equipment	(535)	(2,276)
Proceeds from sale of property and equipment	0	6
Purchases of investments	(69,421)	0
Net cash used in investing activities	(69,956)	(2,270)
Cash flows from financing activities
Proceeds from initial public offering, net of underwriting discount, commissions and offering costs paid	109,352	(233)
Proceeds from long-term debt	0	15,000
Proceeds from issuance of common stock	2,590	656
Non-controlling interest	0	1
Net cash provided by financing activities	113,757	15,424
Net change in cash, cash equivalents and restricted cash	14,191	(8,541)
Cash, cash equivalents and restricted cash, beginning of year	25,300	33,841
Cash, cash equivalents and restricted cash, end of year	39,491	25,300
Supplemental disclosure of cash flow information
Cash paid for interest	4,234	2,738
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	32	6
Landlord paid tenant improvements	0	794
Unpaid deferred offering costs	0	717
Right-of-use asset obtained in exchange for lease obligation	3,982
Net exercise of redeemable convertible preferred stock warrants to preferred stock	37,121	0
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	144,140	0
Redeemable convertible preferred stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	1,784	0
Common stock warrants outstanding
Cash flows from financing activities
Proceeds from exercise of warrants	$ 31	$ 0

Summary of Significant Accounting Policies - Deferred Initial Public Offering Costs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Deferred initial public offering costs	$ 0	$ 950,000
Offering costs	$ 2,481,000	$ 233,000

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Financial Liabilities Measure on a Recurring Basis

The following tables sets forth by level within the fair value hierarchy the Company’s assets and liabilities that are reported at fair value as of December 31, 2019 and 2018, using the inputs defined above (in thousands):


	December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	—		9,919		—		9,919
Corporate bonds/notes	—		10,176		—		10,176
U.S. government securities	—		44,456		—		44,456
Asset-backed securities	—		5,181		—		5,181
	$	34,363	$	69,732	$	—	$	104,095


	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	10,495	$	—	$	—	$	10,495
Liabilities:
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091

Option Pricing Model Assumptions

Additionally, the model’s inputs reflected assumptions that market participants would use in pricing the instrument in a current period transaction and included:


			December 31,
			2018
Time to liquidity (years)			0.57
Expected volatility			62.5%
Discounted cash flow rate			12.0%
Risk-free interest rate			2.6%
Marketability discount rate			14%

Changes in the Redeemable Convertible Warrant Liability




Fair value at December 31, 2017	$	4,185
Change in fair value recorded in other income (expense), net	11,906
Fair value at December 31, 2018	16,091
Change in fair value recorded in other income (expense), net	21,030
Reclassification upon IPO	(37,121)
Fair Value at December 31, 2019	$	—

Quarterly Financial Information (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Financial Information

The following table provides selected unaudited quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data):


	Three Months Ended
	December 31, 2019		September 30, 2019		June 30, 2019		March 31, 2019
Revenue	$	18,634	$	17,026	$	14,928	$	12,766
Gross profit	$	13,913	$	12,856	$	11,231	$	9,427
Net loss	$	(8,291)	$	(8,007)	$	(11,959)	$	(24,158)
Net loss per share, basic and diluted	$	(0.27)	$	(0.26)	$	(0.42)	$	(20.12)


	Three Months Ended
	December 31, 2018		September 30, 2018		June 30, 2018		March 31, 2018
Revenue	$	11,470	$	9,614	$	7,767	$	5,706
Gross profit	$	7,803	$	6,732	$	5,376	$	3,772
Net loss	$	(15,618)	$	(8,953)	$	(7,651)	$	(5,408)
Net loss per share, basic and diluted	$	(14.12)	$	(8.49)	$	(8.16)	$	(7.31)

Stock Option Plans

12 Months Ended

Dec. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock Option Plans

In 2007, the Company established its 2007 Stock Option Plan which provided for the granting of stock options to employees, directors and consultants of the Company. In connection with its acquisition of NeuroCo in December 2018, the Company also assumed NeuroCo’s 2015 Equity Incentive Plan. In March 2019, the Company's Board of Directors approved the termination of the 2007 Stock Option Plan and the NeuroCo 2015 Equity Incentive Plan and the adoption of the 2019 Equity Incentive Plan, or the 2019 Plan, which became effective immediately prior to the Company's IPO. The 2019 Plan provides for the grant of ISOs to employees and for the grant of NSOs, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to employees, directors and consultants. A total of 2,317,000 shares of common stock were initially reserved for issuance pursuant to the 2019 Plan. In addition, the shares reserved for issuance under the 2019 Plan will also include shares reserved but not issued under the 2007 Stock Option Plan, plus any share awards granted under the 2007 Stock Option Plan that expire or terminate without having been exercised in full or that are forfeited or repurchased. In addition, the number of shares available for issuance under the 2019 Plan will also include an annual increase on the first day of each fiscal year beginning in fiscal 2020, equal to the lesser of (i) 3,000,000 shares; (ii) 4.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors. As of December 31, 2019, the Company has reserved 2,366,251 shares of common stock for issuance under the 2019 Plan.

The exercise price of ISOs and NSOs shall not be less than 100% and 85% of the estimated fair value of the shares on the date of grant, respectively, as determined by the Board of Directors. The exercise price of ISOs and NSOs granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant as determined by the Board of Directors. To date, options have a term of ten years and generally vest over 4 years from date of grant.

Activity under the Company’s 2007 Stock Option Plan, NeuroCo 2015 Equity Incentive Plan and 2019 Plan is set forth below:


		Options Outstanding
	Shares Available for Grant	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)





Balances, December 31, 2017	328,290	4,308,890	$	3.09	7.81	$	5,073
Authorized	223,664
Options granted	(629,716)	629,716	$	6.51
Options exercised	—	(438,578)	$	1.50
Options cancelled	135,651	(135,651)	$	1.56
Balances, December 31, 2018	57,889	4,364,377	$	3.79	7.36	$	33,132
Authorized	2,317,000
Options granted	(848,023)	848,023	$	22.77
Options exercised	—	(873,786)	$	1.77
Options cancelled	27,824	(27,824)	$	8.71
Balances December 31, 2019	1,554,690	4,310,790	$	7.91	7.27	$	140,234
Vested and exercisable at December 31, 2019		2,514,891	$	4.27	6.38	$	90,854
Vested and expected to vest at December 31, 2019		4,310,790	$	7.91	7.27	$	140,234

The aggregate intrinsic value of options exercised during the years ended December 31, 2019 and 2018 was $24,867,000 and $787,000, respectively. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying options and the estimated fair value of the common stock on the date of exercise. The weighted-average grant date fair value of options granted during the years ended December 31, 2019 and 2018 was $10.17 and $2.93 per share, respectively. The total fair value of options vested during the years ended December 31, 2019 and 2018 was $2,221,000 and $569,000, respectively, based on the grant date fair value.

The following table summarizes information about stock options outstanding and vested as of December 31, 2019:


	Options Outstanding							Options Vested
Exercise Price	Options Outstanding	Weighted Average Remaining Contractual Term (in Years)	Weighted Average Exercise Price		Number Exercisable	Weighted Average Exercise Price
$1.35 - $3.16	2,072,765	5.86	$	1.84	1,770,060	$	1.71
$4.73 - $8.27	919,355	8.23	$	5.81	383,077	$	5.72
$11.29 - $20.00	1,192,804	8.73	$	16.63	352,383	$	14.46
$36.47 - $48.53	125,866	9.69	$	40.50	9,371	$	45.57
	4,310,790	7.27	$	7.91	2,514,891	$	4.27

2019 Employee Stock Purchase Plan

In March 2019, the Company's Board of Directors adopted the 2019 Employee Stock Purchase Plan, or the 2019 ESPP, under which eligible employees are permitted to purchase common stock at a discount through payroll deductions. A total of 434,000 shares of common stock are reserved for issuance and will be increased on the first day of each fiscal year, beginning in 2020, by an amount equal to the lesser of (i) 1,200,000 shares (ii) 1.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors. The price of the common stock purchased will be the lower of 85% of the fair market value of the common stock at the beginning of an offering period or at the end of a purchase period. The 2019 ESPP was effective upon adoption by the Company's Board of Directors but was not in use until the completion of the Company's IPO in April 2019. The 2019 ESPP is intended to qualify as an "employee stock purchase plan" within the meaning of Section 423 of the Internal Revenue Code of 1986, as amended.

As of December 31, 2019, 61,648 shares of common stock have been issued to employees participating in the 2019 ESPP and 372,352 shares were available for future issuance under the 2019 ESPP.

Stock-Based Compensation

The Company estimated the fair value of stock options using the Black–Scholes option pricing model. The fair value of employee and nonemployee stock options is being amortized on a straight–line basis over the requisite service period of the awards. The fair value of employee and nonemployee stock options was estimated using the following assumptions for the years ended December 31, 2019 and 2018:


	Year Ended December 31,
	2019	2018
Expected term (in years)	5.00 - 6.25	5.00 - 6.25
Expected volatility	42.4% - 42.9%	38.0% - 38.8%
Risk-free interest rate	1.47% - 2.54%	2.68% - 2.98%
Dividend yield	—%	—%

Prior to completion of the Company's IPO, the fair value of common stock was determined by the Company’s Board of Directors, who considered, among other things, contemporaneous valuations of the Company’s common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. For stock options granted after the completion of the IPO, the fair value of the underlying common stock is based on the closing price of the Company's common stock on The NASDAQ Global Market on the date of grant. The expected term of stock options represents the weighted-average period the stock options are expected to remain outstanding. The Company does not have sufficient historical exercise and post-vesting termination activity to provide accurate data for estimating the expected term of options and has opted to use the “simplified method,” whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option. The expected stock price volatility assumption was determined by examining the historical volatilities for industry peers, as the Company does not have sufficient trading history for the Company’s common stock. The Company will continue to analyze the historical stock price volatility and expected term assumption as more historical data for the Company’s common stock becomes available. The risk-free interest rate assumption is based on the U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options. The expected dividend assumption is based on the Company’s history and expectation of dividend payouts.

Effective January 1, 2018, the Company made an accounting policy election to account for forfeitures as they occur.

The fair value of the shares to be issued under the Company’s 2019 ESPP was estimated using the Black-Scholes valuation model with the following assumptions for the year ended December 31, 2019:


						Year Ended December 31,
						2019
Expected term (in years)						0.50 - 0.63
Expected volatility						44.4% - 47.8%
Risk-free interest rate						1.58% - 2.45%
Dividend yield						—%

Total stock-based compensation expense relating to the Company’s stock options and 2019 ESPP during the years ended December 31, 2019 and 2018, is as follows (in thousands):


	Year Ended December 31,
	2019		2018
Cost of goods sold	$	179	$	51
Research and development expenses	426		256
Selling, general and administrative expenses	2,372		604
	$	2,977	$	911

As of December 31, 2019, there was total unrecognized compensation costs of $8,539,000 related to stock options, which are expected to be recognized over a period of approximately 3.04 years and $219,000 related to the 2019 ESPP, which the Company will recognize over 0.39 years.

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Fair Value of the Available-For-Sale Investments

The fair value of the Company's available-for-sale investments as of December 31, 2019 are as follows (in thousands):


	December 31, 2019
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	9,919		—		—		9,919
Corporate bonds/notes	10,180		—		(4)		10,176
U.S. government securities	44,450		9		(3)		44,456
Asset-backed securities	5,181		—		—		5,181
	$	104,093	$	9	$	(7)	$	104,095

Classified as:
Cash equivalents							$	34,363
Short-term investments							51,508
Long-term investments							18,224
							$	104,095

Fair Value of Cash Equivalents, Short-Term and Long-Term Equivalents

The following table summarizes the fair value of the Company’s cash equivalents, short-term and long-term investments classified by maturity as of December 31, 2019 (in thousands):


	December 31,
	2019
Amounts maturing within one year	$	85,871
Amounts maturing after one year through two years	18,224
	$	104,095

Available-For-Sale Investments in Unrealized Loss Position

The following table presents the Company's available-for-sale investments that were in an unrealized loss position as of December 31, 2019 (in thousands):


	December 31, 2019
	Less than 12 months
Assets:	Fair Value		Unrealized Loss
Corporate bonds/notes	$	10,128	$	(4)
U.S. government securities	19,067		(3)
	$	29,195	$	(7)

Schedule of Inventories

Inventories

Components of inventories were as follows (in thousands):


	December 31,
	2019		2018
Raw materials	$	1,203	$	1,054
Finished products	9,119		4,690
	$	10,322	$	5,744

Property, Plant and Equipment

Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2019		2018
Furniture and fixtures	$	657	$	517
Equipment	1,295		1,217
Software	18		76
Leasehold improvements	1,991		1,978
	3,961		3,788
Less: Accumulated depreciation and amortization	(1,550)		(946)
Add: Construction-in-progress	323		38
	$	2,734	$	2,880

Schedule of Accrued Liabilities

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):


	December 31,
	2019		2018
Accrued payroll and related expenses	$	9,151	$	5,157
Provision for sales returns	2,419		1,862
Accrued professional services	682		1,014
Operating lease liability	769		—
Accrued royalty expense	470		313
Deferred revenue	304		137
Accrued travel expenses	431		270
Accrued clinical expenses	241		244
Accrued other expenses	567		451
	$	15,034	$	9,448

Stockholders' Equity (Deficit)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Stockholders' Equity (Deficit)

Redeemable Convertible Preferred Stock

The Company had the following redeemable convertible preferred stock issued and outstanding at December 31, 2018 (in thousands except share amounts):


	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

Redeemable Convertible Preferred Stock Warrants

Stockholders' Equity (Deficit)

Preferred Stock

Common Stock

At December 31, 2019 and 2018, the Company had reserved common stock for future issuances as follows:


	December 31,
	2019	2018
Conversion of Series A redeemable convertible preferred stock	—	1,629,629
Conversion of Series A-1 redeemable convertible preferred stock	—	1,111,111
Conversion of Series B redeemable convertible preferred stock	—	6,264,470
Conversion of Series C redeemable convertible preferred stock and warrants	—	15,064,405
Exercise of options under stock plan	4,310,790	4,364,377
Issuance of options under stock plan	1,554,690	57,889
Issuance of common stock under employee stock purchase plan	372,352	—
Warrants to purchase common stock	—	7,527
	6,237,832	28,499,408

Common Stock Warrants

Quarterly Financial Information (unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information (unaudited)

The following table provides selected unaudited quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data):


	Three Months Ended
	December 31, 2019		September 30, 2019		June 30, 2019		March 31, 2019
Revenue	$	18,634	$	17,026	$	14,928	$	12,766
Gross profit	$	13,913	$	12,856	$	11,231	$	9,427
Net loss	$	(8,291)	$	(8,007)	$	(11,959)	$	(24,158)
Net loss per share, basic and diluted	$	(0.27)	$	(0.26)	$	(0.42)	$	(20.12)


	Three Months Ended
	December 31, 2018		September 30, 2018		June 30, 2018		March 31, 2018
Revenue	$	11,470	$	9,614	$	7,767	$	5,706
Gross profit	$	7,803	$	6,732	$	5,376	$	3,772
Net loss	$	(15,618)	$	(8,953)	$	(7,651)	$	(5,408)
Net loss per share, basic and diluted	$	(14.12)	$	(8.49)	$	(8.16)	$	(7.31)

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenue	$ 63,354	$ 34,557
Cost of goods sold	15,927	10,874
Gross profit	47,427	23,683
Operating expenses:
Research and development	12,272	10,258
Selling, general and administrative	63,220	34,820
Total operating expenses	75,492	45,078
Loss from operations	(28,065)	(21,395)
Interest income	1,656	189
Interest expense	(4,952)	(4,361)
Other income (expense), net	(21,054)	(12,063)
Nett loss	(52,415)	(37,630)
Net loss attributable to non-controlling interest	0	1
Net loss attributable to Silk Road Medical, Inc. common stockholders	(52,415)	(37,629)
Other comprehensive loss:
Unrealized gain on investments, net	2	0
Net change in other comprehensive loss	2	0
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$ (52,413)	$ (37,629)
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (in USD per share)	$ (2.28)	$ (39.16)
Weighted average common shares used to compute net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (in shares)	22,956,679	960,882

Balance Sheet Components - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,961	$ 3,788
Less: Accumulated depreciation and amortization	(1,550)	(946)
Property and equipment, net	2,734	2,880
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	657	517
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,295	1,217
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	18	76
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,991	1,978
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 323	$ 38

Stock Option Plans - Stock Option Assumptions (Details)	12 Months Ended
Stock Option Plans - Stock Option Assumptions (Details)	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	42.40%	38.00%
Expected volatility, maximum	42.90%	38.80%
Risk-free interest rate, minimum	1.47%	2.68%
Risk-free interest rate, maximum	2.54%	2.98%
Dividend yield	0.00%	0.00%
2019 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	44.40%
Expected volatility, maximum	47.80%
Risk-free interest rate, minimum	1.58%
Risk-free interest rate, maximum	2.45%
Dividend yield	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years	5 years
Minimum \| 2019 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months
Maximum \| 2019 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	7 months 17 days

Stockholders' Equity (Deficit) - Narrative (Details)	12 Months Ended
Stockholders' Equity (Deficit) - Narrative (Details)	Dec. 31, 2019 USD ($) vote $ / shares shares	Apr. 30, 2019 $ / shares	Dec. 31, 2018 $ / shares shares
Class of Warrant or Right [Line Items]
Preferred shares authorized (in shares)	5,000,000		0
Preferred shares (in USD per share) \| $ / shares	$ 0.001		$ 0.001
Shares authorized (in shares)	100,000,000		29,879,220
Par value (in USD per share) \| $ / shares	$ 0.001		$ 0.001
Shares issued (in shares)	31,255,267		1,135,310
Shares outstanding (in shares)	31,255,267		1,135,310
Dividends declared \| $	$ 0
Number of votes entitled per share of common stock \| vote	1
IPO
Class of Warrant or Right [Line Items]
Stock issued, price per share (in USD per share) \| $ / shares	$ 20.00	$ 20.00
Warrants to Purchase Common Stock
Class of Warrant or Right [Line Items]
Warrants outstanding (in shares)	0		7,527
Common stock warrants outstanding \| IPO
Class of Warrant or Right [Line Items]
Shares of common stock converted (in shares)	5,968

Acquisition of Variable Interest Entity - NeuroCo	12 Months Ended
Acquisition of Variable Interest Entity - NeuroCo	Dec. 31, 2019
Business Combinations [Abstract]
Acquisition of Variable Interest Entity - NeuroCo	Acquisition of Variable Interest Entity - NeuroCo In December 2014, the Board of Directors of the Company approved the sale of certain intellectual property of Silk Road Medical, Inc., to a newly incorporated entity, NeuroCo, Inc. In consideration for the intellectual property, a promissory note was executed between the two parties for the principal sum of $498,000 with an interest rate of 2.74% per annum, payable on the earlier of 10 years from the date of promissory note, or upon the occurrence of an event of default. The intellectual property transfer was recorded at its carrying value of zero as of December 31, 2014. During 2015 NeuroCo issued $154,000 in common stock to stockholders of the Company. During the year ended December 31, 2018, NeuroCo issued common stock upon the exercise of stock options. These common stock issuance amounts, as they are related to non-controlling investors, were reported as non-controlling interests in subsidiary in the Company’s financial statements and are offset by NeuroCo losses consolidated by the Company. Additionally, NeuroCo incurred research and development related expenses paid for by the Company which were added in to the original promissory note. The Company had identified NeuroCo as a VIE of which the Company is the primary beneficiary. Pursuant to the accounting guidance for consolidating VIEs the main consideration was given to the fact that the amount of total equity investment at risk is not sufficient to permit NeuroCo to finance its activities without additional subordinated financial support. Additionally, NeuroCo and Silk Road Medical had the same Board of Directors and senior management composition, determining the Company to have the power to direct the activities that most significantly impact NeuroCo’s economic performance and the obligation to absorb losses and the right to receive benefits. Accordingly, the financial results of NeuroCo were included in the Company’s financial statements. On December 17, 2018, the Company and NeuroCo entered into the Agreement and Plan of Merger (the “Merger Agreement”) pursuant to which the Company acquired all assets and assumed all liabilities of NeuroCo (the “Merger”). The Merger closed on the same day (the “Closing”) and was consummated through a stock-for-stock transaction based on the relative values of the Company’s and NeuroCo’s equity. In consideration for 100% equity interest of NeuroCo, the Company issued 33,462 shares of its common stock and the above promissory note in the amount of approximately $1,600,000 as of the Closing was settled and canceled. As a result of the Merger, NeuroCo merged into the Company with the Company being the surviving corporation. As the Company already controlled and consolidated NeuroCo and retained the control over NeuroCo’s business after the Merger, the Company accounted for the acquisition of equity interest in NeuroCo as an equity transaction. Therefore, the Company did not recognize a gain or loss in its net loss or comprehensive loss for acquisition of NeuroCo. As the carrying amount of the non-controlling interest as of the Closing was zero, the Company recorded the consideration paid as a decrease to the Company’s additional paid-in capital within stockholder’s equity (deficit). As part of the Merger, the Company assumed NeuroCo’s 2015 Equity Incentive Plan (the “NeuroCo Plan”) along with all of NeuroCo’s rights and obligations under the NeuroCo Plan, except that the number of shares and exercise price of the assumed options have been adjusted based on the Merger exchange ratio of the Company’s common stock and NeuroCo’s common stock. Similarly, the Company assumed outstanding warrants to purchase NeuroCo’s common stock such that the number of shares and exercise price of the assumed warrants have been adjusted based on the Merger exchange ratio of the Company’s common stock and NeuroCo’s common stock. The options and warrants to purchase shares of the Company’s common stock were fully vested upon issuance, as they were replacing fully vested options and warrants to purchase NeuroCo common stock.

Long-term Debt - Term Loan Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017	Oct. 31, 2015
Debt Instrument [Line Items]
Term loan agreement amount	$ 15,000,000
Debt drawn down			$ 58,357,000
Minimum liquidity amount			3,000,000
Minimum net revenue target			$ 30,000,000	$ 15,000,000	$ 5,000,000	$ 1,000,000
Equity cure period			90 days
Long-term debt			$ 44,879,000
Scenario, Forecast
Debt Instrument [Line Items]
Minimum net revenue target		$ 40,000,000
Term Loan
Debt Instrument [Line Items]
Term loan agreement amount	$ 25,000,000							$ 30,000,000
Stated interest rate	10.75%
Interest rate paid-in-kind	2.75%
Interest rate paid in cash	8.00%
Percentage of outstanding loans	25.00%
Market capitalization (greater than)	$ 250,000,000
Term Loan, Tranche A
Debt Instrument [Line Items]
Term loan agreement amount								$ 20,000,000
Financing fee								1.75%
Facility fee								5.00%
Term Loan, Tranche B
Debt Instrument [Line Items]
Term loan agreement amount								$ 10,000,000
Debt drawn down							$ 5,000,000

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Balance Sheet Information

The disclosure impact of the adoption of ASC 842 on the balance sheet was as follows (in thousands):


Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

Balance sheet information as of December 31, 2019 consists of the following (in thousands):


Operating Lease:	December 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,400
Operating lease liability in accrued liabilities	$	769
Operating lease liability in other liabilities	3,700
Total operating lease liabilities	$	4,469

Operating Lease Maturities

The following table summarizes the Company's operating lease maturities as of December 31, 2019 (in thousands):


Year Ending December 31:	Amount
2020	$	1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,230
Less: imputed interest	(761)
Present value of lease liabilities	$	4,469

Minimum Future Lease Payment Previously Disclosed Under ASC 840

The aggregate future minimum lease payments under ASC 840 as of December 31, 2018 were as follows (in thousands):


Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
	$	5,999

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Preparation

The accompanying financial statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Adjustment to Prior Period Financial Statements

Principles of Consolidation

As of December 31, 2017, the consolidated financial statements of the Company include the accounts of Silk Road Medical, Inc. and its consolidated variable interest entity (“VIE”). Disclosure regarding the Company’s participation in the VIE is included in Note 12, “Variable Interest Entity – NeuroCo”. On December 17, 2018, the Company acquired all assets and assumed all liabilities of its VIE. All intercompany balances and transactions have been eliminated in consolidation.

Use of Estimates

Fair Value of Financial Instruments

Cash, Cash Equivalents and Restricted Cash


	December 31,
	2019		2018
Cash and cash equivalents	$	39,181	$	24,990
Restricted cash	310		310
Total cash, cash equivalents and restricted cash	$	39,491	$	25,300

Restricted cash as of December 31, 2019 and 2018 consists of a letter of credit of $310,000 representing collateral for the Company’s facility lease.

Investments

A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the investment.

Concentration of Credit Risk, and Other Risks and Uncertainties

Accounts Receivable

Inventories

Property and Equipment

Deferred Public Offering Costs

Specific incremental legal, accounting and other fees and costs directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. As of December 31, 2018, there were $950,000 of offering costs primarily consisting of legal and accounting fees that were capitalized in other non-current assets on the balance sheet of which $233,000 had been paid. No deferred offering costs were capitalized as of December 31, 2019.

Impairment of Long-Lived Assets

The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If indicators of impairment exist, an impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the assets and their eventual disposition are less than their carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of the long-lived assets exceeds their fair value. The Company did not record any impairment of long-lived assets during the years ended December 31, 2019 and 2018.

Leases

The disclosure impact of the adoption of ASC 842 on the balance sheet was as follows (in thousands):


Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

Redeemable Convertible Preferred Stock Warrant Liability

Redeemable Convertible Preferred Stock

Revenue Recognition

(i)identify the contract(s) with a customer;

(ii)identify the performance obligations in the contract;

(iii)determine the transaction price;

(iv)allocate the transaction price to the performance obligations in the contract; and

(v)recognize revenue when (or as) the entity satisfies a performance obligation.

Under ASC 606, assuming all other revenue recognition criteria have been met, the Company will recognize revenue earlier for arrangements where the Company has satisfied its performance obligations

Cost of Goods Sold

Research and Development

Clinical Trials

Advertising Costs

The Company expenses advertising costs as incurred. Advertising costs include design and production costs, including website development, physician and patient testimonial videos, written media campaigns, and other items. Advertising costs of $362,000 and $186,000 were expensed during the years ended December 31, 2019 and 2018, respectively.

Foreign Currency

Stock–Based Compensation

Income Taxes

Comprehensive Loss

Comprehensive loss consists of net loss and changes in unrealized gains and losses on investments classified as available-for-sale. For the year ended December 31, 2019, the Company’s unrealized gains and losses on available-for-sale investments represent the only component of other comprehensive loss that are excluded from the reported net loss and that are presented in the statements of operations and comprehensive income. Accumulated other comprehensive loss is presented in the accompanying balance sheets as a component of stockholders' equity (deficit). For the year ended December 31, 2018, there was no difference between the Company's comprehensive loss and its net loss.

Net Loss per Share Attributable to Common Stockholders

stock participate in any dividends declared by the Company and are therefore considered to be participating securities.

Net loss per share was determined as follows (in thousands, except share and per share data):


	Year Ended December 31,
	2019		2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(52,415)	$	(37,629)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	22,956,679		960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(2.28)	$	(39.16)


	December 31,
	2019	2018
Redeemable convertible preferred stock outstanding	—	21,233,190
Redeemable convertible preferred stock warrants outstanding	—	2,672,502
Common stock options	4,310,790	4,364,377
Common stock warrants outstanding	—	7,527
Total	4,310,790	28,277,596

Segment and Geographical Information

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents, investments, and the Company's previously outstanding preferred stock warrants. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value:

•Level 1 – quoted prices in active markets are identical assets and liabilities;

•Level 2 – observable inputs other than quotes prizes in active markets for identical assets and liabilities;

•Level 3 – unobservable inputs.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. The Company’s cash equivalents are classified within

Level 1 of the fair value hierarchy because they are valued using quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. The corporate bonds/notes, commercial paper, asset-backed securities and U.S. government securities are classified as Level 2 as they are valued based upon quoted market prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets.


	December 31, 2019
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	—		9,919		—		9,919
Corporate bonds/notes	—		10,176		—		10,176
U.S. government securities	—		44,456		—		44,456
Asset-backed securities	—		5,181		—		5,181
	$	34,363	$	69,732	$	—	$	104,095


	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	10,495	$	—	$	—	$	10,495
Liabilities:
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091

As a derivative liability, the redeemable convertible warrants were initially recorded at fair value and were subject to remeasurement at each balance sheet date. Any change in fair value as a result of a remeasurement was recognized as a component of other income (expense), net in the statements of operations and comprehensive loss. The Company’s redeemable convertible warrant liability was classified within Level 3 of the fair value hierarchy.

At December 31, 2018, the fair value of the redeemable convertible warrant liability was determined by using an option pricing model to allocate the total enterprise value to the various securities within the Company’s capital structure. The fair value of the redeemable convertible warrant liability was based on both the estimated fair value of the Company’s common stock of $11.29 as of December 31, 2018 and on valuation models discounted at current implied market rates which are based on Level 3 inputs.

Additionally, the model’s inputs reflected assumptions that market participants would use in pricing the instrument in a current period transaction and included:


			December 31,
			2018
Time to liquidity (years)			0.57
Expected volatility			62.5%
Discounted cash flow rate			12.0%
Risk-free interest rate			2.6%
Marketability discount rate			14%

The final fair value of the redeemable convertible warrants was remeasured on the date of the Company's initial public offering in April 2019. The final fair value of the redeemable convertible warrant liability was based on the fair value of the Company's common stock at the time of its initial public offering. The following table provides a reconciliation of the beginning and ending balances of the Company's redeemable convertible warrant liability are summarized below (in thousands):




Fair value at December 31, 2017	$	4,185
Change in fair value recorded in other income (expense), net	11,906
Fair value at December 31, 2018	16,091
Change in fair value recorded in other income (expense), net	21,030
Reclassification upon IPO	(37,121)
Fair Value at December 31, 2019	$	—

There were no transfers between fair value hierarchy levels during the years ended December 31, 2019 and 2018.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll and related expenses	$ 9,151		$ 5,157
Provision for sales returns	2,419		1,862
Accrued professional services	682		1,014
Operating lease liability	769
Accrued royalty expense	470		313
Deferred revenue	304		137
Accrued travel expenses	431		270
Accrued clinical expenses	241		244
Accrued other expenses	567		451
Accrued liabilities	$ 15,034	$ 721	$ 9,448

Summary of Significant Accounting Policies - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Net loss attributable to Silk Road Medical, Inc. common stockholders									$ (52,415)	$ (37,629)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted (in shares)									22,956,679	960,882
Net loss per share, basic and diluted (in USD per share)	$ (0.27)	$ (0.26)	$ (0.42)	$ (20.12)	$ (14.12)	$ (8.49)	$ (8.16)	$ (7.31)	$ (2.28)	$ (39.16)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock Issued and Outstanding

The Company had the following redeemable convertible preferred stock issued and outstanding at December 31, 2018 (in thousands except share amounts):


	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

Balance Sheet Components - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Weighted average days to maturity	291 days
Work-in-process inventories	$ 0	$ 0
Depreciation and amortization	$ 712,000,000	$ 517,000,000

Long-term Debt (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Future Maturities Under the Term Loan Agreement

Future maturities under the term loan agreement as of December 31, 2019 are as follows (in thousands):


Year Ending December 31:	Amount
2020	$	4,454
2021	4,442
2022	48,256
	57,152
Add: Accretion of closing fees	1,205
	58,357
Less: Amount representing interest	(13,339)
Less: Amount representing debt discount and debt issuance costs	(139)
Present value of minimum payments	$	44,879

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement of Stockholders' Equity [Abstract]
Offering costs	$ 2,481	$ 233

Summary of Significant Accounting Policies - ASC 842 Impact on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets	$ 3,644	$ 3,982	$ 1,029
Accrued liabilities	15,034	721	9,448
Other liabilities	$ 3,700	4,469	1,069
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets			0
Accrued liabilities			139
Other liabilities			$ 1,069
Restatement Adjustment \| Adjustments Due to ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets		3,982
Accrued liabilities		582
Other liabilities		$ 3,400

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Components of Income Tax Expense (Benefit)

The components of income before taxes are as follows (in thousands):


	Year Ended December 31,
	2019		2018
United States	$	(52,415)	$	(37,630)
International	—		—
	$	(52,415)	$	(37,630)

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the statutory U.S. federal rate to the Company’s effective tax rate is as follows (in thousands):


	Year Ended December 31,
	2019		2018
Tax at federal statutory rate	$	(11,007)	$	(7,902)
State taxes, net of federal benefit	(2,270)		(1,582)
Permanent differences	(4,731)		289
Loss on Series C warrant liability	5,330		2,500
Change in valuation allowance	12,797		6,197
General business credits	(319)		136
Other	208		376
Provision for income taxes	$	8	$	14

Schedule of Deferred Tax Assets and Liabilities

Significant components of the Company’s net deferred tax assets as of December 31, 2019 and 2018 consist of the following (in thousands):


	December 31,
	2019		2018
Deferred tax assets:
Net operating loss carryforwards	$	44,574	$	33,815
Research and development credits	5,287		4,944
Capitalized start-up costs/Intangibles	11		16
Accruals and reserves	2,227		1,169
Property and equipment	188		82
Stock-based compensation	568		274
Operating lease liability	1,133		—
Total deferred tax assets	53,988		40,300
Less: Valuation allowance	(53,126)		(40,300)
Deferred tax liabilities:
Operating lease asset	(862)		—
Total deferred tax liabilities	(862)		—
Net deferred tax assets	$	—	$	—

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the unrecognized tax benefits from January 1, 2018 to December 31, 2019 is as follows (in thousands):


	December 31,
	2019		2018
Balance at the beginning of year	$	1,348	$	615
Increases related to current years’ tax positions	88		118
Increases/(decreases) related to prior years’ tax positions	—		615
Balance at end of year	$	1,436	$	1,348

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 18,634	$ 17,026	$ 14,928	$ 12,766	$ 11,470	$ 9,614	$ 7,767	$ 5,706	$ 63,354	$ 34,557
Gross Profit	13,913	12,856	11,231	9,427	7,803	6,732	5,376	3,772	47,427	23,683
Net loss	$ (8,291)	$ (8,007)	$ (11,959)	$ (24,158)	$ (15,618)	$ (8,953)	$ (7,651)	$ (5,408)	$ (52,415)	$ (37,630)
Net loss per share, basic and diluted (in USD per share)	$ (0.27)	$ (0.26)	$ (0.42)	$ (20.12)	$ (14.12)	$ (8.49)	$ (8.16)	$ (7.31)	$ (2.28)	$ (39.16)

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669625-108580

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4J
-URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591551-111686

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3000-108585

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 10: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4K
-URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591552-111686

+ Details

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Unbilled receivables	$ 102	$ 128

Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 44,574	$ 33,815
Research and development credits	5,287	4,944
Capitalized start-up costs/Intangibles	11	16
Accruals and reserves	2,227	1,169
Property and equipment	188	82
Stock-based compensation	568	274
Operating lease liability	1,133
Total deferred tax assets	53,988	40,300
Less: Valuation allowance	(53,126)	(40,300)
Operating lease asset	(862)
Total deferred tax liabilities	(862)	0
Net deferred tax assets	$ 0	$ 0

Commitments and Contingencies - Balance Sheet Information (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating lease right-of-use asset in other non-current assets	$ 3,400
Operating lease liability	769
Operating lease liability in other liabilities	3,700
Total operating lease liabilities	$ 4,469

Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 39,181	$ 24,990
Restricted cash	310	310
Total cash, cash equivalents and restricted cash	39,491	25,300	$ 33,841
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 310	$ 310

Fair Value Measurements - Fair Value Assumptions (Details) - Recurring - Level 3	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Time to liquidity (years)	6 months 25 days
Expected volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.625
Discounted cash flow rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.120
Risk-free interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.026
Marketability discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.14

Long-term Debt - Future Maturities Under the Term Loan (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Debt Disclosure [Abstract]
2020	$ 4,454
2021	4,442
2022	48,256
Long-term debt, gross before accretion of closing fees	57,152
Add: Accretion of closing fees	1,205
Long-term debt, gross	58,357
Less: Amount representing interest	(13,339)
Less: Amount representing debt discount and debt issuance costs	(139)
Present value of minimum payments	$ 44,879

Recent Accounting Pronouncements	12 Months Ended
Recent Accounting Pronouncements	Dec. 31, 2019
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Standards In February 2016, the FASB issued ASU No. 2016-02, "Leases," that supersedes ASC 840, "Leases." Subsequently, the FASB issued several updates to ASU No. 2016-02, codified in ASC Topic 842 (“ASC 842”). The Company adopted ASC 842 on January 1, 2019 using the modified retrospective method for all leases not substantially completed as of the date of adoption. The Company elected to apply the package of practical expedients, which allowed the Company to not reassess: (i) whether expired or existing contracts contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs for any existing lease. Recently Issued Accounting Standards In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Statements. This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement, which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. ASU 2018-13 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Cloud Computing Arrangements, which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. ASU 2018-15 became effective for the Company on January 1, 2020. The Company does not believe that the adoption of this new guidance will have a material impact on its financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which enhances and simplifies various aspects of the income tax accounting guidance related to intraperiod tax allocation, interim period accounting for enacted changes in tax law, and the year-to-date loss limitation in interim period tax accounting. ASU 2019-12 also amends other aspects of the guidance to reduce complexity in certain areas. ASU 2019-12 will become effective for the Company on January 1, 2021. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its financial statements and related disclosures.

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2019
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Shares Authorized (in shares)	0	24,069,615
Shares Issued (in shares)	0	21,233,190
Shares Outstanding (in shares)	0	21,233,190
Preferential Liquidation Value (in thousands)	$ 0	$ 121,144,000
Carrying Value (in thousands)	$ 0	$ 105,235,000
Series A
Temporary Equity [Line Items]
Shares Authorized (in shares)		1,629,629
Shares Issued (in shares)		1,629,626
Shares Outstanding (in shares)		1,629,626
Per share Preference (in USD per share)		$ 2.70
Preferential Liquidation Value (in thousands)		$ 4,400,000
Carrying Value (in thousands)		$ 4,369,000
Series A-1
Temporary Equity [Line Items]
Shares Authorized (in shares)		1,111,111
Shares Issued (in shares)		1,111,109
Shares Outstanding (in shares)		1,111,109
Per share Preference (in USD per share)		$ 3.38
Preferential Liquidation Value (in thousands)		$ 3,755,000
Carrying Value (in thousands)		$ 3,723,000
Series B
Temporary Equity [Line Items]
Shares Authorized (in shares)		6,264,470
Shares Issued (in shares)		6,264,463
Shares Outstanding (in shares)		6,264,463
Per share Preference (in USD per share)		$ 6.11
Preferential Liquidation Value (in thousands)		$ 38,276,000
Carrying Value (in thousands)		$ 38,014,000
Series C
Temporary Equity [Line Items]
Shares Authorized (in shares)		15,064,405
Shares Issued (in shares)		12,227,992
Shares Outstanding (in shares)		12,227,992
Per share Preference (in USD per share)		$ 6.11
Preferential Liquidation Value (in thousands)		$ 74,713,000
Carrying Value (in thousands)		$ 59,129,000

Commitments and Contingencies - Lease Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Renewal term	5 years
Operating lease costs	$ 870
Operating lease liabilities	$ 721
Weighted average discount rate	6.50%
Weighted average remaining lease term	4 years 9 months 29 days

Stockholders' Equity (Deficit) (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Reserved Common Stock for Future Issuances

At December 31, 2019 and 2018, the Company had reserved common stock for future issuances as follows:


	December 31,
	2019	2018
Conversion of Series A redeemable convertible preferred stock	—	1,629,629
Conversion of Series A-1 redeemable convertible preferred stock	—	1,111,111
Conversion of Series B redeemable convertible preferred stock	—	6,264,470
Conversion of Series C redeemable convertible preferred stock and warrants	—	15,064,405
Exercise of options under stock plan	4,310,790	4,364,377
Issuance of options under stock plan	1,554,690	57,889
Issuance of common stock under employee stock purchase plan	372,352	—
Warrants to purchase common stock	—	7,527
	6,237,832	28,499,408

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (SX 210.5-02(29))
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Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
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-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21488-112644

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section S99
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-Subparagraph (SX 210.5-02(27))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 14: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

Balance Sheet Components

12 Months Ended

Dec. 31, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Balance Sheet Components

Investments

The fair value of the Company's available-for-sale investments as of December 31, 2019 are as follows (in thousands):


	December 31, 2019
			Gross Unrealized						Estimated
	Amortized Cost		Gains		Losses		Fair Value
Money market funds	$	34,363	$	—	$	—	$	34,363
Commercial paper	9,919		—		—		9,919
Corporate bonds/notes	10,180		—		(4)		10,176
U.S. government securities	44,450		9		(3)		44,456
Asset-backed securities	5,181		—		—		5,181
	$	104,093	$	9	$	(7)	$	104,095

Classified as:
Cash equivalents							$	34,363
Short-term investments							51,508
Long-term investments							18,224
							$	104,095

The following table summarizes the fair value of the Company’s cash equivalents, short-term and long-term investments classified by maturity as of December 31, 2019 (in thousands):


	December 31,
	2019
Amounts maturing within one year	$	85,871
Amounts maturing after one year through two years	18,224
	$	104,095

Available-for-sale investments held as of December 31, 2019 had a weighted average days to maturity of 291 days.

The following table presents the Company's available-for-sale investments that were in an unrealized loss position as of December 31, 2019 (in thousands):


	December 31, 2019
	Less than 12 months
Assets:	Fair Value		Unrealized Loss
Corporate bonds/notes	$	10,128	$	(4)
U.S. government securities	19,067		(3)
	$	29,195	$	(7)

Inventories

Components of inventories were as follows (in thousands):


	December 31,
	2019		2018
Raw materials	$	1,203	$	1,054
Finished products	9,119		4,690
	$	10,322	$	5,744

As of December 31, 2019 and 2018, there were no work-in-process inventories.

Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):


	December 31,
	2019		2018
Furniture and fixtures	$	657	$	517
Equipment	1,295		1,217
Software	18		76
Leasehold improvements	1,991		1,978
	3,961		3,788
Less: Accumulated depreciation and amortization	(1,550)		(946)
Add: Construction-in-progress	323		38
	$	2,734	$	2,880

Depreciation and amortization expense was $712,000 and $517,000 for the years ended December 31, 2019 and 2018, respectively.

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):


	December 31,
	2019		2018
Accrued payroll and related expenses	$	9,151	$	5,157
Provision for sales returns	2,419		1,862
Accrued professional services	682		1,014
Operating lease liability	769		—
Accrued royalty expense	470		313
Deferred revenue	304		137
Accrued travel expenses	431		270
Accrued clinical expenses	241		244
Accrued other expenses	567		451
	$	15,034	$	9,448

Summary of Significant Accounting Policies - Advertising Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Advertising expense	$ 362	$ 186

Stock Option Plans - Information by Exercise Price (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	4,310,790	4,364,377	4,308,890
Weighted Average Remaining Contractual Term (in Years)	7 years 3 months 7 days	7 years 4 months 9 days	7 years 9 months 21 days
Weighted Average Exercise Price (in USD per share)	$ 7.91	$ 3.79	$ 3.09
Number Exercisable (in shares)	2,514,891
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 4.27
Exercise Price One
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	2,072,765
Weighted Average Remaining Contractual Term (in Years)	5 years 10 months 9 days
Weighted Average Exercise Price (in USD per share)	$ 1.84
Number Exercisable (in shares)	1,770,060
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 1.71
Exercise Price One \| Minimum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	1.35
Exercise Price One \| Maximum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	$ 3.16
Exercise Price Two
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	919,355
Weighted Average Remaining Contractual Term (in Years)	8 years 2 months 23 days
Weighted Average Exercise Price (in USD per share)	$ 5.81
Number Exercisable (in shares)	383,077
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 5.72
Exercise Price Two \| Minimum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	4.73
Exercise Price Two \| Maximum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	$ 8.27
Exercise Price Three
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	1,192,804
Weighted Average Remaining Contractual Term (in Years)	8 years 8 months 23 days
Weighted Average Exercise Price (in USD per share)	$ 16.63
Number Exercisable (in shares)	352,383
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 14.46
Exercise Price Three \| Minimum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	11.29
Exercise Price Three \| Maximum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	$ 20.00
Exercise Price Four
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	125,866
Weighted Average Remaining Contractual Term (in Years)	9 years 8 months 8 days
Weighted Average Exercise Price (in USD per share)	$ 40.50
Number Exercisable (in shares)	9,371
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 45.57
Exercise Price Four \| Minimum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	36.47
Exercise Price Four \| Maximum
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price (in USD per share)	$ 48.53

Stockholders' Equity (Deficit) - Shares Reserved for Future Issuance (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	6,237,832	28,499,408
Warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	7,527
Exercise of options under stock plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	4,310,790	4,364,377
Issuance of options under stock plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	1,554,690	57,889
Issuance of common stock under employee stock purchase plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	372,352	0
Conversion of Series A redeemable convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,629,629
Conversion of Series A-1 redeemable convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	1,111,111
Conversion of Series B redeemable convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	6,264,470
Conversion of Series C redeemable convertible preferred stock and warrants
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	0	15,064,405