Stockholders' Equity Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Fair Value of Stock Option

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2020			2019			2018
Risk-free interest rate		1.26	%		2.64	%		2.39	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		73.3	%		88.4	%		76.0	%
Expected dividend yield		0	%		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2019		8,310,683		$	10.36		7.01	$	3,427
Granted		5,095,365			7.11
Exercised		(2,214,011	)		6.51
Forfeited		(1,154,907	)		10.50
Outstanding as of December 31, 2020		10,037,130		$	9.54		7.87	$	156,627
Options exercisable as of December 31, 2020		3,164,129		$	14.70		5.75	$	34,652

Summary of Restricted Stock Activity

The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2020:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2019		130,000		$	8.86
Granted		6,500		$	25.36
Forfeited		(5,000	)	$	9.12
Vested		(125,000	)	$	2.29
Unvested restricted stock units as of December 31, 2020		6,500		$	25.36

Summary of Stock Based Compensation Expense

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss:

	Year Ended December 31,
	2020		2019		2018
Research and development expenses	$	4,760	$	4,613	$	8,388
General and administrative expenses		4,064		3,731		8,253
	$	8,824	$	8,344	$	16,641