Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2022

Share-Based Payment Arrangement [Abstract]

Schedule of total employee, director and non-employee stock-based compensation expense recognized

Total employee, director and non-employee stock-based compensation expense recognized was as follows:

	Year Ended December 31,
(in thousands)	2022		2021		2020
General and administrative	$	17,281	$	12,813	$	10,769
Research and development	31,632		24,162		20,850
	$	48,913	$	36,975	$	31,619

	Year Ended December 31,
(in thousands)	2022		2021		2020
Stock options	$	29,758	$	27,909	$	26,045
ESPP	1,174		992		804
RSUs	17,981		8,074		4,770
	$	48,913	$	36,975	$	31,619

Schedule of stock options outstanding

Information with respect to stock options outstanding is as follows:

	December 31,
	2022		2021		2020
Exercisable options	6,679,948		5,576,430		4,668,179
Weighted average exercise price per share of exercisable options	$	26.99	$	24.15	$	21.75
Weighted average grant date fair value per share of options granted during the year	$	15.45	$	21.65	$	16.96
Options available for future grants	3,622,319		3,597,371		3,346,092
Weighted average remaining contractual life	6.30		6.65		7.00

Summary of stock option activity

The following table summarizes stock option activity for the years ended December 31, 2022 and 2021:

	Number of Shares	Weighted- Average Exercise Price (Per Share)(1)		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)(2)
Balances at December 31, 2019	7,174,319	$	24.03	7.32	$	79,116
Options granted	1,679,324	33.08
Options forfeited	(243,384)	32.93
Options exercised(3)	(858,470)	19.36
Balances at December 31, 2020	7,751,789	26.23		7.00	$	134,941
Options granted	1,827,234	41.22
Options forfeited	(382,454)	36.15
Options exercised(3)	(520,240)	23.61
Balances at December 31, 2021	8,676,329	29.11		6.65	$	100,057
Options granted	2,135,233	29.45
Options forfeited	(533,435)	34.09
Options exercised(3)	(195,485)	18.46
Balances at December 31, 2022	10,082,642	$	29.12	6.30	$	27,141
As of December 31, 2022
Options vested and expected to vest	10,082,642	$	29.12	6.30	$	27,141
Exercisable	6,679,948	$	26.99	5.10	$	26,979

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(1)The weighted average exercise price per share is determined using exercise price per share for stock options.

(2)The aggregate intrinsic value is calculated as the difference between the exercise price of the option and the fair value of our common stock for in-the-money options at December 31, 2022 and 2021.

(3)The total intrinsic value of stock options exercised was $1.6 million, $9.2 million, and $16.3 million for the years ended December 31, 2022, 2021 and 2020 respectively.

Schedule of weighted average assumptions used for estimation of fair value of stock options

The fair value of employee stock options was estimated using the following weighted average assumptions for the years ended December 31, 2022, 2021 and 2020:

	Options
	2022	2021	2020
Common stock fair value per share	$19.74 - 38.08	$30.65- 49.47	$20.69 - 45.91
Expected volatility	51.51% - 54.36%	53.91% - 56.82%	52.93% - 58.95%
Risk-free interest rate	1.57% - 4.34%	0.47% - 1.33%	0.29% - 1.71%
Expected dividend yield	—	—	—
Expected term (in years)	6.00 - 7.65	6.00 - 7.65	5.23 - 7.65

Schedule of weighted average assumptions used for estimation of fair value of ESPP

The fair value of employee stock options was estimated using the following weighted average assumptions for the years ended December 31, 2022, 2021 and 2020:

	ESPP
	2022	2021	2020
Expected term (years)	0.5 - 2.0	0.5 - 2.0	0.5 - 2.0
Expected volatility	43.19% - 55.72%	46.08% - 66.37%	50.77% - 66.37%
Risk-free interest rate	0.13% - 4.72%	0.04% - 1.65%	0.09% - 1.65%
Expected dividend yield	—	—	—

Summary of restricted stock unit activity

The following table summarizes RSU activity for the years ended December 31, 2022:

	Number of Shares	Weighted- Average Grant Date Fair Value (Per Unit)
Unvested at December 31, 2019	90,006	$	34.66
Granted	348,288	32.51
Vested	(62,355)	32.61
Forfeited	(17,114)	32.33
Unvested at December 31, 2020	358,825	$	33.04
Granted	670,700	39.11
Vested	(151,555)	32.76
Forfeited	(51,822)	36.68
Unvested at December 31, 2021	826,148	$	37.79
Granted	875,330	29.45
Vested	(341,073)	37.37
Forfeited	(127,854)	33.66
Unvested at December 31, 2022	1,232,551	$	32.41