Stock-Based Compensation

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	5,621	$	5,059	$	10,521	$	9,556
General and administrative		3,950		3,345		7,459		6,188
Total stock-based compensation expense	$	9,571	$	8,404	$	17,980	$	15,744

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 June 30,						Six Months Ended June 30,
	2017			2016			2017			2016
Stock Options
Expected term (in years)		5.4			5.3			5.7			5.3
Expected volatility		71.0	%		71.7	%		71.6	%		69.8	%
Risk-free interest rate		1.9			1.3			2.3			1.4
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	17.06		$	11.60		$	16.26		$	11.48

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 - 77.2		%	69.1 - 80.7		%	52.8 - 77.2		%	61.9 - 80.7		%
Risk-free interest rate	0.5 - 1.3		%	0.2 - 0.9		%	0.5 - 1.3		%	0.2 - 0.9		%
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	9.10		$	10.60		$	8.98		$	10.98