Equity-Based Compensation (Tables)

12 Months Ended

Dec. 31, 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:

	Year Ended December 31,
	2018		2017		2016
	(In thousands)
Research and development	$	8,994	$	7,280	$	4,083
General and administrative		8,052		8,042		2,632
Total	$	17,046	$	15,322	$	6,715

Summary of Restricted Stock Activity

The following table summarizes the Company’s restricted stock activity for the year ended December 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2017		479,822		$	0.90
Granted		86,250			22.98
Vested		(411,372	)		0.83
Cancelled		(45,627	)		8.31
Unvested restricted stock as of December 31, 2018		109,073		$	15.53

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

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

	Year Ended December 31,
	2018			2017			2016
Risk-free interest rate		2.7	%		2.0	%		1.3	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		87.1	%		93.9	%		88.0	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2018:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2017		4,705,448		$	12.09
Granted		2,128,126			20.38
Exercised		(1,142,944	)		9.32
Forfeited		(652,967	)		16.59
Outstanding at December 31, 2018		5,037,663		$	15.63		8.38	$	8,940
Exercisable at December 31, 2018		1,736,487		$	11.42		7.34	$	6,360