Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07

Schedule of stock option activity

		Weighted		Weighted
		average		average	Aggregate
	Number of	exercise		contractual	intrinsic
	shares	price		term	value
				(in years)	(in thousands)
Outstanding as of December 31, 2018	14,784,770	$	8.93	8.89	$	126,367
Granted	3,059,004		12.67
Exercised	(1,449,309)		1.68
Forfeited	(1,290,558)		8.58
Outstanding as of December 31, 2019	15,103,907	$	10.41	8.22	$	36,006
Vested and expected to vest as of December 31, 2019	15,103,907	$	10.41	8.22	$	36,006
Options exercisable as of December 31, 2019	5,182,808	$	6.33	7.49	$	21,743

Schedule of restricted common stock award activity

		Weighted average
		grant‑date
	Shares	fair value
Unvested restricted common stock as of December 31, 2018	1,559,401	$	0.502
Issued	—		—
Vested	(1,258,837)		0.519
Forfeited	(201,250)		0.387
Unvested restricted common stock as of December 31, 2019	99,314	$	0.526

Schedule of restricted stock unit activity

		Weighted average
		grant‑date
	Shares	fair value
Unvested restricted common stock as of December 31, 2018	—	$	—
Issued	277,200		10.660
Vested	—		—
Forfeited	—		—
Unvested restricted common stock as of December 31, 2019	277,200	$	10.660

Schedule of allocation of share based compensation

The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):

	Year ended December 31,
	2019		2018		2017
Research and development expenses	$	9,011	$	3,787	$	1,756
General and administrative expenses		32,260		23,741		16,147
	$	41,271	$	27,528	$	17,903

Employees, directors and non-employees

Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07

Schedule of assumptions used to determine the fair value of stock awards granted

	Year ended December 31,
	2019		2018		2017
Risk-free interest rate	2.07	%	2.71	%	2.05	%
Expected volatility	76.9	%	74.0	%	75.6	%
Expected dividend yield	—		—		—
Expected term (in years)	6.06		6.21		6.39

Market-Based Stock Options

Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07

Schedule of assumptions used to determine the fair value of stock awards granted

	Year ended December 31,
	2018
Risk-free interest rate	3.15	%
Expected volatility	69.0	%
Expected dividend yield	—
Derived service period (in years)	2.30

ASU 2018-07

Fair value of stock-based awards granted to employees, directors, and, in 2018, non-employees, subsequent to the adoption of ASU 2018-07

Schedule of assumptions used to determine the fair value of stock awards granted

	Year ended December 31,
	2017
Risk-free interest rate	2.02% - 2.31%
Expected volatility	74% - 85%
Expected dividend yield	—
Expected term (in years)	6 - 10
Fair value of common stock	$0.19 - $6.28