Financial Instruments and Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Schedule of Financial Assets and Liabilities Measured at Fair Value

The following tables summarize the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy:

	March 31, 2019
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Money market funds	$	111,801	$	—	$	—	$	111,801
Total assets	$	111,801	$	—	$	—	$	111,801

	December 31, 2018
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Money market funds	$	21,680	$	—	$	—	$	21,680
Total assets	$	21,680	$	—	$	—	$	21,680
Liabilities:
Convertible preferred stock warrant liability	$	—	$	—	$	313	$	313
Total liabilities	$	—	$	—	$	313	$	313

Schedule of Change in the Fair Value of the Warrant Liability

The change in the fair value of the warrant liability for the three months ended March 31, 2019 is summarized below (in thousands):

Balance at December 31, 2018	$	313
Change in fair value of warrant liability		609
Net exercise of warrants		(133	)
Conversion of Series A preferred stock warrants to common    stock warrants upon the closing of the IPO		(789	)
Balance at March 31, 2019	$	—

The change in the fair value of the warrant liability for the three months ended March 31, 2018 is summarized below (in thousands):

Balance at December 31, 2017	$	577
Exercise of warrants		(211	)
Expiration of warrants, included in change in fair value of   warrant liability		(87	)
Change in fair value of warrant liability		46
Balance at March 31, 2018	$	325

Schedule of Fair Value of Warrants Determined Using the Black-Scholes Option Pricing Model

The fair value of the warrants was determined using the Black-Scholes option pricing model and the following assumptions:

	Three Months Ended March 31,
	2019		2018
Expected term (in years)		5.28	6.21
Expected volatility	43.9%		42.8%
Risk-free interest rate	2.49%		2.62%
Expected dividend yield	0%		0%