Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

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):

	Quarter Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Research and development	$	4,105	$	4,892	$	12,761	$	5,775
General and administrative		2,739		3,377		7,471		3,959
Total stock-based compensation expense	$	6,844	$	8,269	$	20,232	$	9,734

Schedule of Weighted-average Assumptions used to Estimate Fair Value of Stock Awards

The weighted-average assumptions used to estimate the fair value of stock options using the Black-Scholes option valuation model and the resulting weighted average fair value of stock options granted were as follows:

	Quarter Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Stock Options
Expected term (in years)		5.2			5.0			5.2			5.0
Expected volatility		70	%		63	%		70	%		63	%
Risk-free interest rate		1.7			1.7			1.7			1.7
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	13.47		$	9.22		$	16.21		$	9.22

Employee Stock Purchase Plans
Expected term (in years)		—			—			1.3			—
Expected volatility		—	%		—	%		65	%		—	%
Risk-free interest rate		—			—			0.3			—
Expected dividend yield		—			—			—			—
Weighted average estimated fair value	$	—		$	—		$	9.75		$	—