Equity-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

8. Equity-Based Compensation

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

Restricted Stock

Restricted stock is measured at fair value based on the quoted price of the Company’s common stock.

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

As of December 31, 2018, there was $1.4 million of unrecognized equity-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.6 years.

Stock Options

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $15.05 per option for options granted during the year ended December 31, 2018, $12.43 per option for options granted during the year ended December 31, 2017 and $6.48 per option for options granted during the year ended December 31, 2016. 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	%

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

As of December 31, 2018, there was $38.2 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 2.9 years.