Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Recognized Stock-Based Compensation Expense

Stock-based compensation expense recognized during the years ended December 31, 2016, 2015 and 2014 was as follows:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Research and development	$	11,197	$	5,924	$	1,093
General and administrative		11,823		9,316		1,419
	$	23,020	$	15,240	$	2,512

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, 2016, 2015 and 2014 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,
	2016			2015			2014
Expected dividend yield		0	%		0	%		0	%
Expected volatility		80.15	%		90.54	%		98.86	%
Risk-free interest rate		1.47	%		1.59	%		1.95	%
Expected life of option	6.05 years			6.03 years			6.38 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, 2015		3,002,809		$	26.67		8.67	$	96,479
Granted		1,454,568			35.98
Exercised		(124,903	)		8.09
Forfeited		(100,667	)		44.76
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Vested and expected to vest as of December 31, 2016		3,727,837		$	29.36		8.21	$	84,395
Vested and exercisable as of December 31, 2016		1,702,819		$	24.11		7.59	$	48,096

Summary of Activity Relating to Restricted Stock

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2015		42,781
Issued		—
Vested		(42,781	)
Forfeited		—
Repurchased		—
Outstanding as of December 31, 2016		—