Stock-based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Stock-Based Compensation Expense by Award Type

The Company’s stock-based compensation expense by award type was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Stock options	$	6,965	$	4,309	$	19,261	$	11,248
Restricted stock units		69		69		206		206
Employee stock purchase plan		128		105		512		301
	$	7,162	$	4,483	$	19,979	$	11,755

Stock-Based Compensation Expense

The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	3,657	$	1,940	$	9,530	$	5,364
General and administrative		3,505		2,543		10,449		6,391
	$	7,162	$	4,483	$	19,979	$	11,755

Summary of Unvested RSUs Activity Under 2015 Plan

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

			Weighted-
			average
			Grant Date
	Shares		Fair Value
Unvested balance at December 31, 2018		40	$	20.90
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested balance at September 30, 2019		40	$	20.90

2014 and 2015 Equity Incentive Plan [Member]

Summary of Stock Option Activity

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2018		4,855		$	19.31		7.6	$	118,185
Granted		1,596		$	47.82
Exercised		(684	)	$	8.07
Cancelled or forfeited		(237	)	$	33.74
Outstanding at September 30, 2019		5,530		$	28.31		7.7	$	69,769
Exercisable at September 30, 2019		2,827		$	15.48		6.6	$	60,340
Vested and expected to vest at September 30, 2019		5,530		$	28.31		7.7	$	69,769

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.