Stock-Based Compensation

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

5.	Stock-Based Compensation

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

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