Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2017

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

Summary of Activity Related to Stock Option Awards

A summary of the activity related to stock option awards during the year ended December 31, 2017 is as follows:

	Shares Subject to Options		Weighted-Average Exercise Price per Share		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2017	1,879,428		$	4.34
Granted	1,099,806		$	24.44
Exercises	(199,191	)	$	4.92
Forfeitures and cancellations	(354,140	)	$	13.72
Outstanding at December 31, 2017	2,425,903		$	12.03	7.39	$	215,142.6
Exercisable at December 31, 2017	1,131,354		$	3.23	5.77	$	110,292.8

Summary of Weighted Average Assumptions in Stock Option Valuations

The estimated fair values of stock option awards granted to employees were determined on the date of grant using the Black-Scholes option valuation model with the following weighted average assumptions:

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate	2.0		%	1.4		%	1.4		%
Expected volatility	64.3		%	70.5		%	71.2		%
Expected dividend yield	—		%	—		%	—		%
Expected term (in years)	6.25			6.25			6.10
Weighted average grant date fair value per share	$	14.82		$	4.35		$	4.48

Summary of Non-cash Stock-based Compensation Expense

Total non-cash stock-based compensation expense for all stock awards that was recognized in the consolidated statements of operations and comprehensive loss is as follows:

	Year Ended December 31,
(in thousands)	2017		2016		2015
Research and development	$	1,347	$	420	$	282
General and administrative	3,031		740		322
Total	$	4,378	$	1,160	$	604