Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Assumptions Used to Calculate Fair Value of Options Granted to Employees

The fair value of stock options granted to employees during the years ended December 31, 2017, 2016 and 2015 was estimated at the date of grant using the following assumptions:

	Years Ended December 31,
	2017	2016	2015
Risk-free interest rate	1.7 – 2.2%	1.1 – 2.0%	1.5 – 1.8%
Expected dividend yield	0%	0%	0%
Expected term	5.0 – 6.1 years	5.0 – 5.8 years	5.0 – 5.9 years
Expected volatility	64 – 68%	67 – 69%	66 – 67%

Schedule of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense during the years ended December 31, 2017, 2016 and 2015 as follows:

	Years Ended December 31,
(in thousands)	2017		2016		2015
Employee awards:
Research and development expense	$	6,902	$	6,083	$	7,711
General and administrative expense		5,886		4,685		5,392
Restructuring expense		—		1,371		—
Stock-based compensation expense for employee awards		12,788		12,139		13,103
Stock-based compensation expense for non-employee    awards		—		1		58
Total stock-based compensation expense	$	12,788	$	12,140	$	13,161

Summary of Stock Option Activity

The following table summarizes stock option activity during the year ended December 31, 2017:

(in thousands, except per share amounts)	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2016		1,902		$	57.70		5.97	$	7,564
Granted		2,111		$	23.69
Exercised		(323	)	$	20.70
Forfeited		(2,074	)	$	56.60
Outstanding at December 31, 2017		1,616		$	22.07		6.65	$	60,031
Vested and expected to vest at December 31,    2017		1,615		$	22.07		6.64	$	60,031
Exercisable at December 31, 2017		996		$	26.43		4.96	$	59,689