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Stock award plan - (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of stock option activity

A summary of stock option activity is as follows:

    

    

    

Weighted-

    

  

Weighted-

average

Aggregate

average

remaining

intrinsic

Number of

exercise

contractual

value(in 

options

price

term

thousands)

 

Outstanding at December 31, 2018

 

8,534,358

$

28.58

 

  

 

  

Granted

 

3,977,995

$

35.81

 

  

 

  

Exercised

 

(949,887)

$

19.25

 

  

 

  

Forfeited

 

(518,527)

$

35.27

 

  

 

  

Outstanding at December 31, 2019

 

11,043,939

$

31.67

 

  

 

  

Granted

 

2,777,975

$

51.06

 

  

 

  

Exercised

 

(3,268,452)

$

24.25

 

  

 

  

Forfeited

 

(889,785)

$

42.14

 

  

 

  

Outstanding at December 31, 2020

 

9,663,677

$

38.72

 

  

 

  

Granted

 

2,487,234

61.36

 

  

 

  

Exercised

 

(635,871)

28.01

 

  

 

  

Forfeited/Cancelled

 

(742,458)

52.04

 

  

 

  

Outstanding at December 31, 2021

 

10,772,582

$

43.66

 

6.91

years

$

50,564

Vested or Expected to vest at December 31, 2021

 

4,409,301

$

51.05

 

8.34

years

$

7,022

Exercisable at December 31, 2021

 

5,958,347

$

37.53

 

5.74

years

$

43,249

Schedule of assumptions used to estimate fair values of grants made on the date of grant

The fair values of grants made in the years ended December 31, 2021, 2020 and 2019 were contemporaneously estimated on the date of grant using the following assumptions:

2021

    

2020

    

2019

Risk-free interest rate

0.51% - 1.24%

 

0.34% - 1.45%

 

1.58% - 2.63%

Expected volatility

74% - 89%

 

87% - 89%

 

62% - 92%

Expected term

5.5 years

 

5.75 years

 

5.75 - 6.11 years

Summary of information on the Company's restricted stock

The following table summarizes information on the Company’s restricted stock awards and units:

Restricted Stock Awards and Units

Weighted

Average

Grant

Number of

Date

    

Shares

    

Fair Value

Unvested at December 31, 2020

982,058

$

41.78

Granted

 

1,018,228

62.54

Vested

 

(335,114)

36.09

Forfeited

 

(145,341)

57.71

Unvested at December 31, 2021

 

1,519,831

$

55.43

Schedule of share-based compensation expense recorded in the statement of operations

The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows:

Year ended December 31, 

2021

    

2020

    

2019

Research and development

$

53,632

$

38,716

$

20,836

Selling, general and administrative

 

49,881

 

31,609

 

21,298

Total

$

103,513

$

70,325

$

42,134