Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Disclosure Of Compensation Related Costs Sharebased 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 and six months ended June 30, 2017 and 2016 was estimated at the date of grant using the following assumptions:

	Three Months Ended June 30,		Six Months Ended June 30,
	2017	2016	2017	2016
Risk-free interest rate	1.7 – 2.0%	1.1 – 1.5%	1.7 – 2.1%	1.5 – 1.8%
Expected dividend yield	0%	0%	0%	0%
Expected term	5.7 – 6.1 years	5.0 – 5.8 years	5.7 – 6.1 years	5.0 – 5.9 years
Expected volatility	65%	67 – 69%	65 – 68%	66 – 67%

Schedule of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense during the three and six months ended June 30, 2017 and 2016 as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2017		2016		2017		2016
Employee awards:
Research and development expense	$	4,916	$	1,465	$	5,383	$	2,991
General and administrative expense		3,818		2,102		4,146		3,086
Total stock-based compensation expense	$	8,734	$	3,567	$	9,529	$	6,077

Summary of Stock Option Activity

The following table summarizes stock option activity during the six months ended June 30, 2017:

						Weighted-Average Remaining		Aggregate
(in thousands, except per share amounts)	Options			Weighted-Average Exercise Price		Contractual Term (in years)		Intrinsic Value
Outstanding at December 31, 2016		19,024		$	5.77		5.97	$	7,564
Granted		19,388		$	2.46
Exercised		(2,562	)	$	2.29
Forfeited		(16,935	)	$	6.24
Outstanding at June 30, 2017		18,915		$	2.43		5.87	$	440
Vested and expected to vest at June 30, 2017		18,870		$	2.43		5.86	$	440
Exercisable at June 30, 2017		12,560		$	2.81		3.98	$	440