Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2021

Equity [Abstract]

Summary of Stock Option Activity

The following table summarizes the stock option activity for the nine months ended September 30, 2021:

	Shares (in thousands)			Weighted- average Exercise Price		Weighted- average Remaining Contractual Term (Years)
Outstanding at December 31, 2020		18,912		$	19.59		7.21
Granted		885		$	14.96
Exercised		(370	)	$	15.29
Expired and forfeited		(1,170	)	$	22.15
Outstanding at September 30, 2021		18,257		$	19.29		6.53

Summary of Restricted Stock Unit Activity

The following table summarizes the restricted stock unit activity for the nine months ended September 30, 2021:

	Shares (in thousands)			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2020		603		$	15.84
Granted		193		$	14.73
Released		(115	)	$	15.99
Forfeited		(40	)	$	15.84
Outstanding at September 30, 2021		641		$	15.48

Summary of Stock-Based Compensation Expense related to Stock-Based Payment Awards Granted

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development	$	4,521	$	4,612	$	14,130	$	14,580
General and administrative		2,836		3,277		8,414		10,012
Sales and marketing		3,879		3,206		11,419		9,591
Total stock-based compensation expense	$	11,236	$	11,095	$	33,963	$	34,183

Summary of Fair Value of Option Grant on Grant Date Using Black-Scholes Option Pricing Model

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Nine Months Ended
	September 30,
	2021			2020
Risk-free interest rate		1.0	%		0.9	%
Dividend yield		0.0	%		0.0	%
Volatility		65.8	%		67.3	%
Expected life (years)		6			6