Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

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,
	2016		2015		2014
Research and development	$	5,778	$	3,623	$	1,562
General and administrative	6,387		4,224		1,682
Total stock-based compensation expense	$	12,165	$	7,847	$	3,244

Schedule of Employee Stock Options Award 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,
	2016	2015	2014
Expected dividend yield	0%	0%	0%
Expected volatility	64% - 69%	73% - 75%	67%
Risk-free interest rate	1.2% - 2.4%	1.6% - 2.1%	1.8% - 2.3%
Expected term	6.25 years	6.25 years	6.25 years

Schedule of Stock Option and Restricted Stock Unit Activity

The following table summarizes stock option and restricted stock unit (RSU) activity for 2016:

	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2015	4,146,064		$	16.90	7.4
Granted	399,949		23.20
Options exercised or RSUs vested	(526,715	)	3.59
Forfeited or expired	(181,238	)	26.53
Outstanding, December 31, 2016	3,838,060		18.93		7.0	$	24,862
December 31, 2016:
Exercisable	2,292,923		13.71		6.1	22,458
Vested and expected to vest	3,651,523		18.57		7.0	24,496