Equity-based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Stock-Based Compensation Expense

The Company uses the straight-line attribution method to recognize stock-based compensation expense for stock options and restricted stock awards. Stock options and restricted stock generally vests over four years with 25% vesting on the first anniversary, and the remaining vesting monthly thereafter. The following table presents stock-based compensation expense in the Company’s Consolidated Statements of Operations: 

	Year Ended December 31,
	2016		2015		2014
Research and development	$	4,848	$	1,924	$	487
General and administrative		5,844		1,760		208
Loss from equity method investment		152		—		—
Total	$	10,844	$	3,684	$	695

Fair Value of Employee and Non-employee Stock Option Award on the Grant Date Using the Black-Scholes Option-Pricing Model

There were no stock options granted prior to 2015. The Company estimated the fair value of each employee and non-employee stock option award on the grant date using the Black-Scholes option-pricing model based on the following assumptions:

	Year Ended December 31
	2016			2015
Employees:
Options granted		2,411,240			1,913,319
Weighted - average exercise price	$	12.19		$	2.32
Weighted-average grant date fair value	$	8.47		$	3.11
Assumptions:
Weighted-average expected volatility		81.0	%		76.4	%
Expected term (in years)		6.0			6.0
Weighted-average risk free interest rate		1.4	%		1.7	%
Expected dividend yield		0.0	%		0.0	%
Non employees:
Options granted		215,710			26,667
Weighted- average exercise price	$	19.54		$	1.85
Weighted- average grant date fair value	$	17.38		$	5.05
Assumptions:
Weighted average expected volatility		88.2	%		84.1	%
Expected term (in years)		10.0			10.0
Weighted-average risk free interest rate		2.4	%		2.2	%
Expected dividend yield		0.0	%		0.0	%

Summary of Stock Option Activity for Employees and Non-employees

The following table summarizes stock option activity for employees and non-employees during the year ended December 31, 2016 (intrinsic value in thousands):

	Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		1,939,986		$	2.31		9.7	$	6,688
Granted		2,626,950		$	12.79
Exercised		(18,900	)		1.81			$	216
Cancelled or forfeited		(12,665	)	4.98
Outstanding at December 31, 2016		4,535,371		$	8.38		9.1	$	53,975
Exercisable at December 31, 2016		960,867		$	3.24		8.8	$	16,361
Vested or expected to vest at    December 31, 2016 (1)		4,169,347		$	8.23		9.1	$	50,155

Summary of the Company's Restricted Stock Activity

The following table summarizes restricted stock activity for employees and non-employees during the year ended December 31, 2016:

	Reflected as outstanding upon vesting			Reflected as outstanding upon grant date			Total			Weighted- Average Grant Date Fair Value
Unvested restricted Common Stock at    December 31, 2015		142,794			1,485,244			1,628,038		$	4.35
Vested		(53,427	)		(834,388	)		(887,815	)		4.78
Unvested restricted Common Stock at    December 31, 2016		89,367			650,856			740,223		$	3.84