Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 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		(3,250	)
Outstanding as of December 31, 2018		82,700

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 years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Research and development	$	50,871	$	19,893	$	11,197
General and administrative		51,092		15,641		11,823
	$	101,963	$	35,534	$	23,020

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:

	2018		2017		2016
	(in thousands)
Stock options	$	100,342	$	34,525	$	22,820
Restricted stock units		651		617		—
Employee stock purchase plan		970		392		166
Restricted stock awards		—		—		34
	$	101,963	$	35,534	$	23,020

Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted

The fair value of each option granted to employees and non-employee directors during the years ended December 31, 2018, 2017 and 2016 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:

	Year Ended December 31,
	2018			2017			2016
Expected dividend yield		0	%		0	%		0	%
Expected volatility		74.45	%		79.89	%		80.15	%
Risk-free interest rate		2.68	%		2.03	%		1.47	%
Expected life of option	6.04 years			6.03 years			6.05 years

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		3,103,494			164.50
Exercised		(767,147	)		34.67
Forfeited		(392,173	)		64.74
Outstanding as of December 31, 2018		7,530,767		$	93.22		8.03	$	227,447
Vested and expected to vest as of December 31, 2018		6,254,429		$	85.55		7.86	$	209,356
Exercisable as of December 31, 2018		2,846,158		$	40.69		6.71	$	159,799