Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2022

Share-Based Payment Arrangement [Abstract]

Summary of Stock-Based Compensation Expense

The following table reflects the components of stock-based compensation expense recognized in our Consolidated Statements of Operations (in millions):

	Year Ended December 31,
	2022		2021		2020
Research and development	$	33	$	29	$	11
General and administrative		32		26		11
Total stock-based compensation	$	65	$	55	$	22

Summary of Restricted Stock Units Activity

We grant restricted stock units (RSUs) to our employees and directors under the 2018 Plan. The RSUs vest annually or quarterly over four years for employees and annually for directors. The following table summarizes information regarding our RSUs for the year ended December 31, 2022:

	Total Restricted Stock Units (in millions)			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2021		1.1		$	32.23
RSUs granted		0.9			30.22
RSUs vested		(0.5	)		32.27
RSUs forfeited or canceled		(0.2	)		30.65
Nonvested at December 31, 2022		1.3		$	31.09

Summary of Information Regarding Stock Options

The following table summarizes information regarding our stock options for the year ended December 31, 2022:

	Shares Subject to Outstanding Options (in millions)			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Outstanding at December 31, 2021		12.0		$	19.46
Options granted		2.3		$	30.37
Options exercised		(1.4	)	$	13.17
Options forfeited or canceled		(0.9	)	$	25.93
Outstanding at December 31, 2022		12.0		$	21.77		7.60	$	50
Options vested and expected to vest as of December 31, 2022		12.0		$	21.77		7.60	$	50
Options exercisable as of December 31, 2022		6.7		$	18.01		7.01	$	42

Schedule of Assumptions Used to Calculate Fair Value and the Weighted-Average Grant Date Fair Value of Stock Options Granted

The following table summarizes the key assumptions used to calculate the fair value and the resulting weighted-average grant date fair value of stock options granted:

	Year Ended December 31,
Stock Options	2022		2021		2020
Weighted average closing price of our common stock on grant date	$	30.37	$	33.03	$	17.36
Risk-free interest rate	2.4% - 4.0%		1.0% - 1.4%		0.4% - 0.5%
Expected term (in years)	6.02		6.02		6.02
Volatility	76.5% - 79.3%		75.3% - 77.6%		76.5% - 78.5%
Dividend yield	0%		0%		0%
Weighted average fair value of stock options granted	$	20.75	$	22.05	$	11.57

	Year Ended December 31,
ESPP	2022	2021	2020
Risk-free interest rate	1.6% - 4.7%	0.0% - 0.6%	0.1% - 0.2%
Expected term (in years)	0.5 - 2.0	0.5-2.0	0.5-2.0
Volatility	68.9% - 82.5%	61.2% - 95.7%	66.6% - 136.0%
Dividend yield	0%	0%	0%