Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

The following is a summary of option activity under the 2015 Plan and the 2005 Plan for the year ended December 31, 2016:

	2015 Plan				2005 Plan
			Weighted-				Weighted-
			Average				Average
	Stock		Exercise		Stock		Exercise
	Options		Price		Options		Price
Outstanding at January 1, 2016		8,357,003	$	8.97		15,450,542	$	3.31
Granted		19,343,937	$	3.37		—	$	—
Exercised		—	$	—		(701,663)	$	2.14
Canceled		(2,596,337)	$	6.85		(620,750)	$	4.80
Outstanding at December 31, 2016		25,104,603	$	4.87		14,128,129	$	3.30
Vested and expected to vest at December 31, 2016		22,625,915	$	5.06		14,075,419	$	3.29
Shares exercisable at December 31, 2016		2,417,574	$	8.50		10,643,754	$	2.86
Shares available for grant at December 31, 2016		5,850,397

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The fair value of stock options granted under the 2015 Plan and 2005 Plan was estimated at the date of grant or the date upon which the 2015 Plan was approved by the Company’s stockholders for stock options granted prior to that time using the Black-Scholes option-pricing model with the following assumptions:

	2016	2015	2014
Weighted average fair value of options granted	$1.88	$4.38	$2.39
Risk-free interest rate	0.97%-1.78%	1.19%-2.13%	1.24%-2.22%
Dividend yield	0%	0%	0%
Volatility	57.86%-108.88%	53.58%-68.39%	52.47%-67.93%
Expected term (in years)	4.22-7.28	3.98-7.34	4.04-6.96
Expected forfeiture rate	0%-16.33%	0%-16.33%	0%-23.15%

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the year ended December 31, 2016:

			Per Share
			Weighted-
			Average
	Number of		Grant-Date
	Shares		Fair Value
Outstanding and Unvested at January 1, 2016		25,000	$	8.72
Restricted stock granted		45,000	$	4.99
Restricted stock vested		—	$	—
Restricted stock forfeited		(25,000)	$	8.72
Outstanding and Unvested at December 31, 2016		45,000	$	4.99

Stock-Based Compensation Expense

The Company recorded stock-based compensation expense for awards issued under the above mentioned plans in the statements of operations as follows (in thousands):

	Years ended December 31,
	2016		2015		2014
Research and development	$	11,168	$	6,771	$	2,843
General and administrative		7,992		6,660		3,247
Total stock-based compensation expense	$	19,160	$	13,431	$	6,090

Employee Stock Purchase Plan [Member]

Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]

Assumptions used to Estimate Grant Date Fair Value of Stock Options granted using Black-Scholes Option-Pricing Model

The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	2016	2015	2014
Range of Black-Scholes fair values of ESPP shares granted	$1.86-$4.76	$1.06-$3.38	$0.78-$2.08
Risk-free interest rate	0.22%-0.61%	0.05%-0.35%	0.04%-0.24%
Dividend yield	0%	0%	0%
Volatility	43.03%-86.75%	40.79%-64.24%	50.80%-67.57%
Expected term (in years)	0.5-2.0	0.5-2.0	0.5-1.5
Expected forfeiture rate	5%	5%	5%