Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock-Based Compensation Expense

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):


	Year Ended December 31,
	2014		2013		2012
Research and development	$	1,562	$	507	$	472
General and administrative		1,682		355		366

Total stock-based compensation expense	$	3,244	$	862	$	838

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:


	Year Ended December 31,
	2014	2013	2012
Expected dividend yield	0%	0%	0%
Expected volatility	67%	53% - 67%	51%
Risk-free interest rate	1.8% - 2.3%	1.2% - 2.2%	1.2%
Expected term.	6.25 years	7 years	7 years

Schedule of Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity for 2014:


	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2013		3,200,958		$	4.90		6.9
Granted		976,869			27.06
Exercised		(568,906	)		1.31
Forfeited or expired		(36,805	)		9.76

Outstanding, December 31, 2014		3,572,116			11.40		7.3	$	84,515

December 31, 2014:
Exercisable		1,668,962			3.24		5.3		53,122
Vested and expected to vest		3,341,192			11.03		7.2		80,323