Stock-Based Compensation (Tables)

12 Months Ended

Dec. 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		393,539
Vested		(82,128	)
Forfeited		(60,868	)
Outstanding as of December 31, 2019		333,243

Summary of Stock-Based Compensation Expense Recognized

Stock-based compensation expense recognized during the years ended December 31, 2019, 2018 and 2017 was as follows:

		2019		2018		2017
	(in thousands)
Research and development		$	62,931	$	50,871	$	19,893
Selling, general and administrative			90,300		51,092		15,641
		$	153,231	$	101,963	$	35,534

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the years ended December 31, 2019, 2018 and 2017 was as follows:

	2019		2018		2017
	(in thousands)
Stock options	$	140,517	$	100,342	$	34,525
Restricted stock units		10,992		651		617
Employee stock purchase plan		1,722		970		392
	$	153,231	$	101,963	$	35,534

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

The fair value of each option granted under the Company’s equity plans has been calculated on the date of grant using the following weighted average assumptions:

	Year Ended December 31,
	2019			2018			2017
Expected dividend yield		0	%		0	%		0	%
Expected volatility		71.34	%		74.45	%		79.89	%
Risk-free interest rate		2.21	%		2.68	%		2.03	%
Expected life of option	6.05 years			6.04 years			6.03 years

Summary of Activity Relating to Time Based and Performance Based Stock Options

The table below summarizes activity related to time-based and performance-based 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		2,215,606			129.15
Exercised		(1,031,989	)		42.72
Forfeited		(551,271	)		138.35
Outstanding as of December 31, 2019		8,163,113		$	106.30		7.75	$	87,972
Vested and expected to vest as of December 31, 2019		7,035,349		$	102.32		7.63	$	84,442
Exercisable as of December 31, 2019		3,737,468		$	78.55		6.63	$	77,652