Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of offering activity

	Number of	Total
	Shares Purchased	Proceeds
Offering Period	(in thousands)
November 1, 2018 - April 30, 2019	132,177	$	2,147
May 1, 2019 - October 31, 2019	136,084		2,176
November 1, 2019 - April 30, 2020	97,247		3,061
May 1, 2020 - October 31, 2020	136,288		4,052

Summary of stock option activity

	Outstanding Options
					Weighted-
			Weighted-		Average
	Shares		Average		Remaining	Aggregate
	Available for	Number of	Exercise		Contractual	Intrinsic
	Grant	Shares	Price		Life	Value
(in thousands, except for contractual life and exercise price)					(in years)
Balance at December 31, 2019	6,416	8,497	$	10.39	6.88	$	197,955
Additional shares authorized	3,120	—	$	—
Options granted	(457)	457	$	27.67
Options exercised	—	(2,092)	$	11.24
Options forfeited/cancelled	155	(155)	$	15.39
RSUs granted (1)	(6,204)
RSUs forfeited/cancelled	436
Balance at December 31, 2020	3,466	6,707	$	11.19	6.04	$	592,468
Exercisable at December 31, 2020		5,080	$	9.04	5.40	$	459,683
Vested and expected to vest at December 31, 2020		6,609	$	11.09	6.01	$	584,396

Schedule of performance-based awards

Period Granted	Options Granted	RSUs Granted	Options Vested	RSUs Vested	Milestone	Valuation Method
(in thousands)
Q1 2019	200	300	138	219	(1)	Monte-Carlo Simulation
Q2 2019	—	188	—	—	(2)	Fair Market Value
Q3 2019	—	50	—	25	(1)	Monte-Carlo Simulation
Q1 2020	150	300	75	150	(1)	Monte-Carlo Simulation
Q1 2020	—	436	—	4	(3)	Fair Market Value
Q1 2020	129	—	—	—	(3)	Black-Scholes-Merton
Q2 2020	—	21	—	—	(3)	Fair Market Value
Q3 2020	10	—	10	—	(4)	Black-Scholes-Merton
Q3 2020	—	27	—	5	(3)	Fair Market Value
Q4 2020	—	32	—	—	(1)	Monte-Carlo Simulation
Q4 2020	—	22	—	—	(5)	Fair Market Value

(1) The awards vest based on the achievement of certain values of the Company’s common stock at multiple thresholds within certain periods and are contingent upon the completion of requisite service through the date of such vesting.

(2) The vesting of the awards will be triggered after the end of the achievement milestone, as measured by the Company.

(3) The awards vest based on achievement of certain revenue targets and are contingent upon the completion of requisite service through the date of such vesting.

(4) The awards vest based on achievement of a reimbursement target.

(5) The awards will vest based on achievement of certain revenue and recruiting targets.

Restricted stock units

		Weighted-
		Average
	Number of	Grant Date
	Shares	Fair Value
	(in thousands)
Balance at December 31, 2019	2,404	$	19.86
Granted	3,102	$	39.94
Vested	(1,100)	$	19.69
Canceled/Forfeited	(218)	$	24.70
Balance at December 31, 2020	4,188	$	34.02

Summary of stock-based compensation expenses

	Year ended December 31,
	2020						2019						2018
	Employee		Non-Employee		Total		Employee		Non-Employee		Total		Employee		Non-Employee		Total
	(in thousands)
Cost of revenues	$	1,691	$	—	$	1,691	$	905	$	32	$	937	$	564	$	5	$	569
Research and development		10,777		647		11,424		5,354		—		5,354		4,043		—		4,043
Selling, general and administrative		36,747		309		37,056		21,730		603		22,333		9,474		112		9,586
Total	$	49,215	$	956	$	50,171	$	27,989	$	635	$	28,624	$	14,081	$	117	$	14,198

Performance-based awards

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of assumptions used in valuation of fair value

	December 31,						December 31,
	2020						2019
Risk-free interest rate		0.54	%	—	1.64	%		1.63	%	—	2.61	%
Expected dividend yield					—						—
Expected volatility		55	%	—	65	%					50	%
Expected term (years)		5.25		—	7.25						7.25

Employee stock options

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of assumptions used in valuation of fair value

	Year ended December 31,
	2020					2019					2018
Expected term (years)	5.22		—	10.00		5.23		—	10.00		5.24		—	5.62
Expected volatility	49.94	%	—	61.96	%	42.53	%	—	45.84	%	40.28	%	—	42.53	%
Expected dividend rate				0	%				0	%				0	%
Risk-free interest rate	0.31	%	—	1.70	%	1.60	%	—	2.60	%	2.37	%	—	3.06	%