Equity-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Equity-Based Compensation Expense

Equity-based compensation expense is classified in the consolidated statements of operations as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
	(In thousands)
Research and development	$	2,591	$	1,987	$	7,617	$	5,048
General and administrative		1,865		1,260		5,847		3,678
Total	$	4,456	$	3,247	$	13,464	$	8,726

Summary of Restricted Stock Activity

The following table summarizes the Company’s restricted stock activity for the nine months ended September 30, 2018:

	Number of Shares			Weighted AverageGrant Date Fair Value per Share
Unvested restricted stock as of January 1, 2018		479,822		$	0.90
Granted		86,250			22.98
Vested		(378,033	)		0.79
Cancelled		(44,524	)		8.48
Unvested restricted stock as of September 30, 2018		143,515		$	12.13

Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted

Key assumptions used to apply this pricing model were as follows:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2018	2017	2018	2017
Risk-free interest rate	2.8%	1.9%	2.6%	1.9%
Expected life of options	6.0 years	6.0 years	5.5-6.25 years	6.0 years
Expected volatility of underlying stock	87.7%	95.4%	88.9%	93.0%
Expected dividend yield	0.0%	0.0%	0.0%	0.0%

Summary of Stock Option Activity

The following is a summary of stock option activity for the nine months ended September 30, 2018:

	Number of Options			Weighted Average ExercisePrice per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at January 1, 2018		4,705,448		$	12.09
Granted		1,076,731			24.43
Exercised		(982,142	)		9.63
Forfeited and Expired		(547,529	)		16.55
Outstanding at September 30, 2018		4,252,508		$	15.20		7.98	$	57,432
Exercisable at September 30, 2018		1,616,167		$	10.49		6.83	$	29,419