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Stock-based compensation (Tables)
 9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Stock-Based Compensation Expense by Award Type Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):
For the three months ended September 30,For the nine months ended September 30,

2020201920202019
Stock options
$22,723 $25,041 $73,236 $73,024 
Restricted stock units
11,935 12,700 37,931 51,593 
Employee stock purchase plan and other4,160 805 12,473 1,381 
$38,818 $38,546 $123,640 $125,998 
Schedule of Stock-Based Compensation Expense by Classification
Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands): 
For the three months ended September 30,For the nine months ended September 30,
2020201920202019
Research and development$18,837 $18,600 $58,204 $63,810 
Selling, general and administrative19,981 19,946 65,436 62,188 
$38,818 $38,546 $123,640 $125,998 
Summary of Stock Option Activity Under Plan
The following table summarizes the stock option activity under the Company’s equity award plans:
Shares
(in thousands)
Weighted-
average
exercise price
per share
Outstanding at December 31, 20195,483 $116.30 
Granted
1,421 $71.50 
Exercised
(55)$24.44 
Canceled, forfeited, or expired
(486)$129.11 
Outstanding at September 30, 20206,363 $106.11 
Exercisable at September 30, 20203,611 $104.72 
Vested and expected to vest at September 30, 20206,363 $106.11 
Summary of Restricted Common Stock Awards The following table summarizes the restricted stock unit activity under the Company’s equity award plans:
Shares
(in thousands)		Weighted-
average
grant date
fair value
Unvested balance at December 31, 20191,127 $146.10 
Granted
936 $71.83 
Vested
(380)$136.07 
Forfeited
(164)$125.57 
Unvested balance at September 30, 20201,519 $105.05