Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

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

The following is a summary of option activity under the 2015 Plan, 2005 Plan and the 1995 Stock Option Plan (“1995 Plan”) for the year ended December 31, 2015:

	2015 Plan				2005 Plan				1995 Plan
	Stock Options		Weighted-Average Exercise Price		Stock Options		Weighted-Average Exercise Price		Stock Options		Weighted-Average Exercise Price
Outstanding at January 1, 2015		—	$	—		16,928,098	$	3.24		35,000	$	2.21
Granted		8,502,941	$	8.97		22,500	$	6.70		—	$	—
Exercised		—	$	—		(1,286,556)	$	2.36		(35,000)	$	2.21
Canceled		(145,938)	$	9.03		(213,500)	$	3.79		—	$	—
Outstanding at December 31, 2015		8,357,003	$	8.97		15,450,542	$	3.31		—	$	—
Vested and expected to vest at December 31, 2015		6,812,836	$	8.97		14,948,571	$	3.25		—	$	—
Shares exercisable at December 31, 2015		280,000	$	8.97		8,423,167	$	2.56		—	$	—
Shares available for grant at December 31, 2015		16,642,997

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:

	2015		2014		2013
Weighted average fair value of options granted		$4.38		$2.39		$1.07
Risk-free interest rate		1.19%-2.13%		1.24%-2.22%		0.54%-1.36%
Dividend yield		0%		0%		0%
Volatility		53.58%-68.39%		52.47%-67.93%		51.55%-73.72%
Expected term (in years)		3.98-7.34		4.04-6.96		3.91-7.05
Expected forfeiture rate		0%-16.33%		0%-23.15%		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, 2015:

	Number of Shares		Per Share Weighted-Average Grant-Date Fair Value
Outstanding and Unvested at January 1, 2015		15,000	$	4.48
Restricted stock granted		25,000	$	8.72
Restricted stock vested		(15,000)	$	4.48
Restricted stock forfeited		—	$	—
Outstanding and Unvested at December 31, 2015		25,000	$	8.72

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,
	2015		2014		2013
Research and development	$	6,771	$	2,843	$	1,262
General and administrative		6,660		3,247		1,218
Total stock-based compensation expense	$	13,431	$	6,090	$	2,480

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:

	2015	2014	2013
Range of Black-Scholes fair values of ESPP shares granted	$1.06-$3.38	$0.78-$2.08	$0.78
Risk-free interest rate	0.05%-0.35%	0.04%-0.24%	0.04%
Dividend yield	0%	0%	0%
Volatility	40.79%-64.24%	50.80%-67.57%	50.80%
Expected term (in years)	0.5-2.0	0.5-1.5	0.5
Expected forfeiture rate	5%	5%	5%