Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

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

Schedule of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense as follows:

	Years ended December 31,
(in thousands)	2015		2014		2013
Employee awards:
Research and development	$	8,271	$	6,864	$	5,954
General and administrative		7,022		6,065		4,808
Stock-based compensation for employee awards		15,293		12,929		10,762
Stock-based compensation for non-employee awards		58		268		(29	)
Total stock-based compensation	$	15,351	$	13,197	$	10,733

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

The fair value of stock options granted to employees during the years ended December 31, 2015, 2014 and 2013 was estimated at the date of grant using the following assumptions:

	Years ended December 31,
	2015	2014	2013
Risk-free interest rate	1.5 - 1.8%	1.6 - 2.0%	0.1 - 1.9%
Expected dividend yield	0%	0%	0%
Expected term	5.0 - 5.9 years	5.0 - 5.9 years	5.3 - 5.9 years
Expected volatility	66 - 67%	64 - 72%	67 - 70%

Summary of Stock Option Activity

The following table summarizes stock option activity during the year ended December 31, 2015:

(in thousands, except per share amounts)	Shares			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2014		19,556		$	4.47		6.17	$	133,599
Granted		3,720		$	9.63
Exercised		(3,606	)	$	2.75
Forfeited		(459	)	$	7.36
Outstanding at December 31, 2015		19,211		$	5.72		6.24	$	47,963
Vested and expected to vest at December 31, 2015		18,923		$	5.68		6.20	$	47,818
Exercisable at December 31, 2015		14,783		$	5.01		5.48	$	44,173