Equity-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Stock-Based Compensation Expense

Stock‑based compensation expense by classification within the consolidated statements of operations and comprehensive income (loss) is as follows (in thousands):

	Years Ended December 31,
	2019		2018		2017
Research and development	$	23,273	$	17,557	$	8,800
General and administrative		20,784		17,428		10,073
Loss from equity method investment		5,467		4,275		1,763
Total	$	49,524	$	39,260	$	20,636

Fair Value of Each Option Issued to Employees and Non-employees Estimated Using Black-Scholes Option-Pricing Model

The fair value of each option issued to employees and non-employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Years Ended December 31,
	2019			2018			2017
Options granted		2,832,784			2,209,597			2,999,847
Weighted-average exercise price	$	39.16		$	51.73		$	16.98
Weighted-average grant date fair value	$	24.57		$	33.82		$	11.16
Assumptions:
Expected volatility		68.9	%		71.9	%		72.5	%
Expected term (in years)		6.0			6.0			6.1
Risk-free interest rate		2.2	%		2.8	%		2.0	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Stock Option Activity

The following table summarizes stock option activity under the Company’s equity award plans (intrinsic value in thousands):

	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		6,689,311		$	25.42		8.3	$	68,572
Granted		2,832,784		$	39.16
Exercised		(1,180,644	)	$	14.91
Cancelled or forfeited		(559,014	)	$	35.35
Outstanding at December 31, 2019		7,782,437		$	31.30		8.2		231,554
Exercisable at December 31, 2019		3,328,398		$	24.07		7.4	$	122,774
Vested and expected to vest at    December 31, 2019		7,782,437		$	31.30		8.2	$	231,554

Summary of Restricted Stock Activity

The following table summarizes the restricted stock activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted- Average Grant Date Fair Value
Unvested restricted common shares at    December 31, 2018		327,342		$	36.72
Granted		503,600			62.11
Vested		(86,758	)		20.38
Cancelled or forfeited		(44,650	)		44.50
Unvested restricted common shares at    December 31, 2019		699,534		$	56.53