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Share-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of Stock Option Activity
The following table summarizes the Company's stock option activity and related information for the year ended December 31, 2020:
 Number of
options
Weighted
average
exercise
price
Weighted-Average Remaining Contractual Term (years)Aggregate Intrinsic Value (millions)
Balance outstanding as of December 31, 20195,287,977 $44.10 
Granted901,094 $115.83 
Exercised(1,318,963)$34.79 
Canceled/forfeited(440,619)$70.64 
Balance outstanding as of December 31, 20204,429,489 $58.82 7.3$712.5 
Options exercisable at December 31, 20202,374,753 $33.62 6.2$441.7 
Schedule of Assumptions Used in Fair Value Estimates The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows:
 Year Ended December 31,
202020192018
Risk-free interest rate1.1%2.2%2.6%
Dividend yield—%—%—%
Volatility factor81.5%82.1%94.3%
Expected term (in years)5.65.66.0
Weighted average estimated fair value per share$77.92$52.03$24.39
Schedule of Share-based Compensation Expense
Total share-based compensation expense by consolidated statement of operations and comprehensive loss classification is presented below (in thousands):
Year ended December 31,
202020192018
Research and development expense$48,044 $31,024 $7,232 
General and administrative expense37,803 24,513 8,622 
$85,847 $55,537 $15,854 
Schedule of Restricted Stock Units Activity A summary of the Company's RSU activity for the year ended December 31, 2020 is as follows:
 Number of
RSUs
Weighted average grant date fair valueAggregate Intrinsic Value (millions)
Balance outstanding as of December 31, 2019— $— 
Granted487,569 $113.91 
Releases(938)$113.20 
Canceled/forfeited(21,371)$109.00 
Balance outstanding as of December 31, 2020465,260 $114.14 $102.2