Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

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, 2017		29,100
Granted		71,400
Vested		(14,550	)
Forfeited		(2,050	)
Outstanding as of September 30, 2018		83,900

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 nine months ended September 30, 2018 and 2017 was as follows:

	Three months ended September 30,				Nine months ended September 30,
	2018		2017		2018		2017
	(in thousands)
Research and development	$	13,997	$	5,399	$	34,991	$	14,239
General and administrative		11,481		4,332		35,332		11,050
	$	25,478	$	9,731	$	70,323	$	25,289

Summary of Stock-Based Compensation Expense by Award Type Recognized

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

	Three months ended September 30,				Nine months ended September 30,
	2018		2017		2018		2017
	(in thousands)
Stock options	$	25,015	$	9,414	$	69,023	$	24,565
Restricted stock units		164		176		487		444
Employee stock purchase plan		299		141		813		280
	$	25,478	$	9,731	$	70,323	$	25,289

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		2,775,594			168.97
Exercised		(711,621	)		35.13
Forfeited		(155,855	)		79.46
Outstanding as of September 30, 2018		7,494,711		$	90.07		8.15	$	459,643
Exercisable as of September 30, 2018		2,460,232		$	36.17		6.73	$	258,528