Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Recognized Stock-Based Compensation Expense

Stock-based compensation expense recognized during the three months ended March 31, 2017 and 2016 was as follows:

	Three Months Ended March 31,
	2017		2016
	(in thousands)
Research and development	$	3,596	$	1,611
General and administrative		2,612		2,103
	$	6,208	$	3,714

Summary of Activity Relating to Stock Options

The table below summarizes activity related to stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Granted		1,370,800			49.74
Exercised		(97,513	)		8.53
Forfeited		(51,632	)		37.12
Outstanding as of March 31, 2017		5,453,462		$	35.27		8.47	$	195,365
Exercisable as of March 31, 2017		1,940,608		$	25.03		7.47	$	89,484