Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Stock-Based Compensation

Schedule of total employee, director and non-employee stock-based compensation expense recognized

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
General and administrative	$	2,804	$	2,008	$	5,095	$	3,862
Research and development		5,427		7,295		9,648		11,297
	$	8,231	$	9,303	$	14,743	$	15,159
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Stock options	$	6,667	$	8,946	$	12,549	$	14,470
ESPP		213		195		407		413
Restricted stock units		1,351		162		1,787		276
	$	8,231	$	9,303	$	14,743	$	15,159

Summary of stock option activity

		Weighted		Weighted
		Average		Average
	Number of	Exercise		Remaining		Aggregate
	Shares Subject	Price		Contractual		Intrinsic
	to Outstanding	(Per		Term		Value
	Options	Share)		(in years)		(in thousands)
Balances at December 31, 2019	7,174,319	$	24.03		7.32
Options granted	1,433,699	$	32.10
Options forfeited	(115,261)	$	31.28
Options exercised	(261,786)	$	18.13
Balances at June 30, 2020	8,230,971	$	25.52		7.33	$	65,362
Exercisable	4,587,110	$	20.28		6.10	$	58,544

Schedule of weighted average assumptions used for estimation of fair value of stock options

	Options				Options
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Expected term (years)	5.7		5.9		6.2		6.0
Expected volatility	56.1	%	61.3	%	54.3	%	61.3	%
Risk-free interest rate	0.41	%	2.07	%	0.84	%	2.43	%
Expected dividend yield	—	%	—	%	—	%	—	%

Schedule of weighted average assumptions used for estimation of fair value of ESPP

	ESPP				ESPP
	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Expected term (years)	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility	50.8 - 62.6	%	55.0 - 71.4	%	50.8 - 62.6	%	55.0 - 71.4	%
Risk-free interest rate	0.18 - 1.65	%	1.47 - 2.70	%	0.18 - 1.65	%	1.47 - 2.70	%
Expected dividend yield	—	%	—	%	—	%	—	%

Summary of restricted stock unity activity

		Weighted
	Restricted	Average Grant
	Stock	Date Fair Value
	Units	(Per unit)
Unvested at December 31, 2019	90,006	$	34.66
Granted	305,794		31.64
Vested	(21,822)		31.91
Forfeited	(7,311)		31.47
Unvested at June 30, 2020	366,667	$	32.37