Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense for Stock Options and Restricted Stock Units Recognized

Stock-based compensation expense for stock options and restricted stock units recognized during the three and nine months ended September 30, 2017 and 2016 was as follows:

	Three months ended September 30,				Nine months ended September 30,
	2017		2016		2017		2016
	(in thousands)
Research and development	$	5,399	$	2,530	$	14,239	$	6,179
General and administrative		4,332		2,218		11,050		6,748
	$	9,731	$	4,748	$	25,289	$	12,927

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		2,169,250			58.33
Exercised		(286,748	)		22.20
Forfeited		(176,132	)		42.57
Outstanding as of September 30, 2017		5,938,177		$	40.35		8.19	$	140,390
Exercisable as of September 30, 2017		2,301,343		$	25.30		7.10	$	85,819