Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees

The fair value of stock options granted to employees during the three months ended March 31, 2016 and 2015 was estimated at the date of grant using the following assumptions:

	Three Months Ended March 31,
	2016		2015
Risk-free interest rate		1.3 – 1.5%		1.6. – 1.8%
Expected dividend yield		0%		0%
Expected term		5.8 years		5.9 years
Expected volatility		67 – 68%		67%

Schedule of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense during the three months ended March 31, 2016 and 2015 as follows:

	Three Months Ended March 31,
(in thousands)	2016		2015
Employee awards:
Research and development	$	1,868	$	1,967
General and administrative		1,665		1,377
Stock-based compensation for employee awards		3,533		3,344
Stock-based compensation for non-employee awards		—		26
Total stock-based compensation	$	3,533	$	3,370

Summary of Stock Option Activity

The following table summarizes stock option activity during the three months ended March 31, 2016:

						Weighted-Average Remaining		Aggregate
(in thousands, except per share amounts)	Shares			Weighted-Average Exercise Price		Contractual Term (in years)		Intrinsic Value
Outstanding at December 31, 2015		19,211		$	5.72		6.24	$	47,963
Granted		3,130		$	5.45
Exercised		(539	)	$	3.12
Forfeited		(155	)	$	7.69
Outstanding at March 31, 2016		21,647		$	5.73		6.65	$	61,405
Vested and expected to vest at March 31, 2016		21,182		$	5.71		6.59	$	60,457
Exercisable at March 31, 2016		14,979		$	5.20		5.50	$	49,061