Fair Value Measurement (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities that Require Fair Value Measurements on a Recurring Basis

The following table summarizes the Company’s assets and liabilities that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands):

			Fair Value Measurements at Reporting Date
	Total		Level 1		Level 2		Level 3
As of September 30, 2018:
Assets
Cash equivalents	$	23,103	$	23,103	$	—	$	—
Liabilities
Preferred stock warrant liability	$	5,990	$	—	$	—	$	5,990
As of December 31, 2017:
Assets
Cash equivalents	$	12,472	$	12,472	$	—	$	—
Liabilities
Derivative liability	$	3,028	$	—	$	—	$	3,028
Preferred stock warrant liability		1,656		—		—		1,656
Total liabilities	$	4,684	$	—	$	—	$	4,684

Weighted-Average Assumptions Used In Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option Pricing Model

The following weighted-average assumptions were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option pricing model as of September 30, 2018 and December 31, 2017:

	September 30,			December 31,
	2018			2017
Expected volatility		68.7	%		68.0	%
Risk-free interest rate		3.0	%		2.2	%
Contractual term (in years)		5.7			5.9
Expected dividend yield		—			—

Estimated Fair Values Per Share of Underlying Redeemable Convertible Preferred Stock

The following estimated fair values per share of the Company’s underlying redeemable convertible preferred stock were used to determine the estimated fair value of the preferred stock warrant liability:

	September 30,		December 31,
	2018		2017
Series AA	$	—	$	5.75
Series B	$	8.95	$	3.76
Series C-1	$	8.95	$	3.76