Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 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

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 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