Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Allocated Stock-Based Compensation Expense

Stock-based compensation expense was allocated to research and development and general and administrative expense as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Research and development	$	5,538	$	5,074	$	16,060	$	14,629
General and administrative		4,090		3,438		11,548		9,627
Total stock-based compensation expense	$	9,628	$	8,512	$	27,608	$	24,256

Schedule of Assumptions used to Estimate Fair Value of Stock Options Granted and Purchases under 2014 Employee Share Purchase Plan

The assumptions used to estimate the fair value of stock options granted and purchases under the Company’s 2014 Employee Share Purchase Plan (“ESPP”) using the Black-Scholes option valuation model were as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Stock Options
Expected term (in years)		5.3			5.3			5.7			5.3
Expected volatility		69.5	%		72.5	%		71.5	%		69.8	%
Risk-free interest rate		1.9			1.2			2.2			1.4
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	26.47		$	10.73		$	16.63		$	11.46

ESPPs
Expected term (in years)	0.5 - 2.0			0.5 - 2.0			0.5 - 2.0			0.5 - 2.0
Expected volatility	52.8 - 76.0		%	63.7 - 80.7		%	52.8 - 77.2		%	61.9 - 80.7		%
Risk-free interest rate	0.6 - 1.3		%	0.4 - 0.9		%	0.5 - 1.3		%	0.2 - 0.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	9.67		$	9.04		$	9.15		$	10.27