Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2015

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

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

The assumptions used to estimate the fair value of options granted to employees and directors at the date of grant for the three and six months ended June 30, 2015 were as follows:

	Three months ended June 30, 2015			Six months ended June 30, 2015
Risk-free interest rate		1.5-1.8	%		1.5-1.8	%
Expected dividend yield		0	%		0	%
Expected term		5.0-5.9 years			5.0-5.9 years
Expected volatility		66-67	%		66-67	%

Schedule of Recognized Stock-Based Compensation Expense

Company recognized stock-based compensation expense as follows for the three and six months ended June 30, 2015 and 2014:

	Three months ended June 30,				Six months ended June 30,
(in thousands)	2015		2014		2015		2014
Employee awards:
Research and development	$	2,369	$	1,841	$	4,335	$	3,522
General and administrative		2,576		2,161		3,954		3,453
Stock-based compensation for employee awards		4,945		4,002		8,289		6,975
Stock-based compensation for non-employee awards		22		153		48		116
Total stock-based compensation	$	4,967	$	4,155	$	8,337	$	7,091

Summary of Stock Option Activity

The following table summarizes stock option activity during the six months ended June 30, 2015:

(in thousands, except per share amounts and years)	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding, December 31, 2014		19,556		$	4.47		6.17	$	133,599
Granted		2,723		$	9.54
Exercised		(2,595	)	$	2.57
Forfeited		(258	)	$	6.82
Outstanding, June 30, 2015		19,426		$	5.40		6.50	$	135,208
Vested and expected to vest, June 30, 2015		19,086		$	5.36		6.45	$	133,696
Exercisable, June 30, 2015		14,210		$	4.66		5.60	$	109,441