Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Based Compensation Expense

The following table summarizes stock-based compensation expense reflected in the consolidated statements of operations (in thousands):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
Research and development	$	988	$	587	$	2,924	$	1,729
General and administrative		1,757		958		4,859		3,001
	$	2,745	$	1,545	$	7,783	$	4,730

Summary of Stock Option Activity

The following table summarizes stock option activity as of September 30, 2018, and changes during the nine months ended September 30, 2018, under the 2007 Long Term Incentive Plan (the 2007 LTIP) and standalone option agreements:

	Number of Options			Weighted- Average Exercise Price
Outstanding at January 1, 2018		3,251,696			19.83
Granted		212,629			35.49
Exercised		(107,524	)		13.91
Forfeited		(17,173	)		23.77
Expired		(2,366	)		30.61
Outstanding at September 30, 2018		3,337,262			20.99
Exercisable at September 30, 2018		1,387,320			18.54