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Stockholders' Equity (Tables)
6 Months Ended
Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Summary of Stock Option Activity

The following table summarizes the stock option activity for the six months ended June 30, 2020:

 

 

 

Shares

(in thousands)

 

 

Weighted-

average

Exercise Price

 

 

Weighted-

average

Remaining

Contractual

Term (Years)

 

Balance at December 31, 2019

 

 

16,665

 

 

$

20.47

 

 

 

7.87

 

Granted

 

 

249

 

 

$

19.24

 

 

 

 

 

Exercised

 

 

(124

)

 

$

10.57

 

 

 

 

 

Expired and forfeited

 

 

(484

)

 

$

22.57

 

 

 

 

 

Balance at June 30, 2020

 

 

16,306

 

 

$

20.46

 

 

 

7.31

 

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

June 30,

 

 

Six Months Ended

June 30,

 

 

 

2020

 

 

2019

 

 

2020

 

 

2019

 

Research and development

 

$

4,681

 

 

$

4,434

 

 

$

9,968

 

 

$

9,758

 

General and administrative

 

 

3,304

 

 

 

3,714

 

 

 

6,735

 

 

 

7,394

 

Sales and marketing

 

 

3,129

 

 

 

4,558

 

 

 

6,385

 

 

 

13,456

 

Total stock-based compensation expense

 

$

11,114

 

 

$

12,706

 

 

$

23,088

 

 

$

30,608

 

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:

 

 

 

June 30,

 

 

 

2020

 

 

2019

 

Risk-free interest rate

 

 

1.1

%

 

 

2.4

%

Dividend yield

 

 

0.0

%

 

 

0.0

%

Volatility

 

 

66.7

%

 

 

68.0

%

Expected life (years)

 

 

6

 

 

 

6

 

1997 Employee Stock Purchase Plan  
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Summary of Fair Value of Option Grant on Grant Date Using Black-Scholes Option Pricing Model

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:

 

 

 

June 30,

 

 

 

2020

 

 

2019

 

Risk-free interest rate

 

 

0.1

%

 

 

2.4

%

Dividend yield

 

 

0.0

%

 

 

0.0

%

Volatility

 

 

63.3

%

 

 

58.4

%

Expected life (months)

 

 

6

 

 

 

6