Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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, 2016		—
Granted		32,500
Vested		—
Forfeited		(3,400	)
Outstanding as of December 31, 2017		29,100

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, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	19,893	$	11,197	$	5,924
General and administrative		15,641		11,823		9,316
	$	35,534	$	23,020	$	15,240

Summary of Stock-Based Compensation Expense by Award Type Recognized

Stock-based compensation expense by award type recognized during the years ended December 31, 2017, 2016 and 2015 was as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Stock options	$	34,525	$	22,820	$	14,796
Restricted stock units		617		—		—
Employee stock purchase plan		392		166		117
Restricted stock awards		—		34		327
	$	35,534	$	23,020	$	15,240

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

The fair value of each option granted to employees and nonemployee directors during the years ended December 31, 2017, 2016 and 2015 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,
	2017			2016			2015
Expected dividend yield		0	%		0	%		0	%
Expected volatility		79.89	%		80.15	%		90.54	%
Risk-free interest rate		2.03	%		1.47	%		1.59	%
Expected life of option	6.03 years			6.05 years			6.03 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, 2016		4,231,807		$	29.99		8.24	$	92,843
Granted		2,360,933			60.59
Exercised		(765,950	)		20.94
Forfeited		(240,197	)		43.61
Outstanding as of December 31, 2017		5,586,593		$	43.58		8.09	$	676,717
Vested and expected to vest as of December 31, 2017		4,504,257		$	42.22		7.98	$	551,729
Exercisable as of December 31, 2017		2,045,229		$	27.95		6.99	$	279,699