Equity-based Compensation (Tables)

6 Months Ended

Jun. 30, 2017

Disclosure of Compensation Related Costs, Share-based 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 awards generally vest over four years with 25% vesting on the first anniversary of service commencement and the remaining 75% vesting monthly thereafter. Effective January 1, 2017, the Company adopted ASU 2016-09, and made an accounting policy election to account for the impact of pre-vesting forfeitures as they occur rather than applying an estimated forfeiture rate, as previously required. The following table presents stock-based compensation expense included in the Company’s Condensed Consolidated Statements of Operations and Comprehensive Loss:

	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development	$	1,946	$	853	$	3,676	$	1,491
General and administrative		1,780		2,648		3,350		3,032
Loss from equity method investment		505		—		951		—
Total	$	4,231	$	3,501	$	7,977	$	4,523

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

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:

	Six Months Ended June 30,
	2017			2016
Employees:
Weighted average expected volatility		72.7	%		77.9	%
Expected term (in years)		6.0			6.0
Risk free interest rate		1.8 - 2.3	%		1.4 - 1.5	%
Expected dividend yield		0.0	%		0.0	%
Non-employees:
Weighted average expected volatility		83.6	%		n/a
Expected term (in years)		10.0			n/a
Risk free interest rate		2.3	%		n/a
Expected dividend yield		0.0	%		n/a

Summary of Stock Option Activity for Employees and Non-employees

The following table summarizes stock option activity for employees and non-employees (intrinsic value in thousands):

	Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2016		4,535,371		$	8.38		9.1	$	53,966
Granted		1,750,048		$	16.13
Exercised		(304,173	)	$	2.40
Cancelled or forfeited		(344,460	)	$	5.11
Outstanding at June 30, 2017		5,636,786		$	11.29		9.0	$	30,510
Exercisable at June 30, 2017		1,319,922		$	5.94		8.4	$	13,309
Vested or expected to vest at June 30, 2017 (1)		5,609,286		$	11.26		9.0	$	30,467

(1) Represents the number of vested options at June 30, 2017 plus the number of unvested options expected to vest in the future.

Summary of the Company's Restricted Stock Activity

The following table summarizes restricted stock activity for employees and non-employees during the six months ended June 30, 2017:

	Reflected as outstanding upon vesting			Reflected as outstanding upon grant date			Total			Weighted- Average Grant Date Fair Value
Unvested restricted common shares as of December 31, 2016		89,367			650,856			740,223		$	3.84
Vested		(16,764	)		(212,075	)		(228,839	)		4.02
Unvested restricted common shares as of June 30, 2017		72,603			438,781			511,384		$	3.76