Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 six months ended June 30, 2017 and 2016 was as follows:

	Three months ended June 30,				Six months ended June 30,
	2017		2016		2017		2016
	(in thousands)
Research and development	$	5,245	$	2,037	$	8,840	$	3,649
General and administrative		4,105		2,428		6,718		4,530
	$	9,350	$	4,465	$	15,558	$	8,179

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		2,070,550			57.10
Exercised		(196,402	)		15.99
Forfeited		(105,882	)		39.06
Outstanding as of June 30, 2017		6,000,073		$	39.64		8.41	$	239,974
Exercisable as of June 30, 2017		2,155,432		$	25.22		7.29	$	117,295