Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Activity Relating to Restricted Stock

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2017		29,100
Granted		71,400
Vested		(14,550	)
Forfeited		(900	)
Outstanding as of June 30, 2018		85,050

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 and six months ended June 30, 2018 and 2017 was as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(in thousands)
Research and development	$	12,095	$	5,245	$	20,994	$	8,840
General and administrative		16,933		4,105		23,851		6,718
	$	29,028	$	9,350	$	44,845	$	15,558

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the three and six months ended June 30, 2018 and 2017 was as follows:

	Three months ended June 30,				Six months ended June 30,
	2018		2017		2018		2017
	(in thousands)
Stock options	$	28,627	$	9,119	$	44,007	$	15,152
Restricted stock units		155		167		324		267
Employee stock purchase plan		246		64		514		139
	$	29,028	$	9,350	$	44,845	$	15,558

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, 2017		5,586,593		$	43.58		8.09	$	676,717
Granted		2,313,844			171.26
Exercised		(490,050	)		36.37
Forfeited		(54,529	)		74.88
Outstanding as of June 30, 2018		7,355,858		$	83.99		8.26	$	575,909
Exercisable as of June 30, 2018		2,439,246		$	34.23		6.89	$	298,310