Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Relating to Restricted Stock Units

The table below summarizes activity relating to restricted stock units:

	Shares
Outstanding as of December 31, 2018		82,700
Granted		354,111
Vested		(13,250	)
Forfeited		(34,091	)
Outstanding as of March 31, 2019		389,470

Summary of Stock-Based Compensation Expense for Stock Options, Restricted Stock Units and Employee Stock Purchase Plan Recognized

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Research and development	$	20,745	$	8,899
General and administrative		23,371		6,918
	$	44,116	$	15,817

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Stock options	$	43,564	$	15,381
Restricted stock units		58		168
Employee stock purchase plan		494		268
	$	44,116	$	15,817

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, 2018		7,530,767		$	93.22		8.03	$	227,447
Granted		1,179,861			155.30
Exercised		(298,102	)		48.32
Forfeited		(130,479	)		120.50
Outstanding as of March 31, 2019		8,282,047		$	103.25		8.10	$	499,599
Exercisable as of March 31, 2019		3,097,197		$	55.80		6.73	$	327,329