Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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,
	2017		2016		2015

Research and development	$	7,388	$	5,778	$	3,623
General and administrative	7,356		6,387		4,224
Total stock-based compensation expense	$	14,744	$	12,165	$	7,847

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,
	2017	2016	2015
Expected dividend yield	0%	0%	0%
Expected volatility	67% - 68%	64% - 69%	73% - 75%
Risk-free interest rate	1.9% - 2.3%	1.2% - 2.4%	1.6% - 2.1%
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 2017:



	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2016	3,838,060		$	18.93	7.0
Granted	1,246,225		20.05
Options exercised or RSUs vested	(253,036	)	2.16
Forfeited or expired	(326,607	)	24.26
Outstanding, December 31, 2017	4,504,642		19.79		7.0	$	18,343

December 31, 2017:
Exercisable	2,910,171		17.65		6.1	17,495
Vested and expected to vest	4,337,015		19.60		6.9	18,275