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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2021
Stock-Based Compensation  
Schedule of summary of option activity The following is a summary of stock options and SARs activity under the 2015 Plan and 2005 Plan for the nine months ended September 30, 2021:
2015 Plan2005 Plan
Stock
Options and SARs		Weighted-Average
Exercise
Price		Stock
Options		Weighted-Average
Exercise
Price
Outstanding at January 1, 20215,420,463 $38.05 214,186 $88.11 
Granted73,091 $176.39 — $— 
Exercised(1,420,550)$32.49 (140,580)$80.03 
Canceled(80,799)$119.36 — $— 
Outstanding at September 30, 20213,992,205 $40.92 73,606 $103.56 
Shares exercisable at September 30, 20211,286,515 $48.62 73,606 $103.56 
Shares available for grant at September 30, 20213,788,799 
Schedule of stock unit activity
The following is a summary of RSUs activity for the nine months ended September 30, 2021:
Number of
Shares		Per Share
Weighted-
Average
Fair Value
Outstanding and Unvested at January 1, 20201,044,980 $72.59 
Restricted stock units granted225,424 $196.93 
Restricted stock units vested(391,960)$57.81 
Restricted stock units forfeited(32,599)$131.95 
Outstanding and Unvested at September 30, 2021845,845 $110.68 
Schedule of stock-based compensation expense
The Company recorded all stock-based compensation expense in the consolidated statements of operations as follows (in thousands):
Three Months Ended
September 30,		Nine Months Ended
September 30,
2021202020212020
Research and development$21,860 $28,730 $70,429 $34,735 
General and administrative23,414 36,975 81,028 42,867 
Total stock-based compensation expense$45,274 $65,705 $151,457 $77,602 
ESPP  
Stock-Based Compensation  
Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model The ESPP is considered compensatory for financial reporting purposes. As such, the fair value of ESPP shares was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
September 30,		Nine Months Ended
 September 30,
2021202020212020
Range of Black-Scholes fair values of ESPP shares granted
$83.47-$152.11
$3.08-$92.67
$83.47-$238.85
$2.57-$92.67
Risk-free interest rate
0.1%-0.2%
0.2%-2.5%
0.1%-0.2%
0.2%-2.6%
Dividend yield—%—%—%—%
Volatility
114.9%-150.6%
77.5%-189.7%
114.9%-159.4%
66.6%-189.7%
Expected term (in years)
0.5-2.0
0.5-2.0
0.5-2.0
0.5-2.0
Stock options  
Stock-Based Compensation  
Schedule of assumptions used to estimate grant date fair value of stock options and stock appreciation rights granted using black-scholes option-pricing model
The fair value of stock options granted under the 2015 Plan was estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions:
Three Months Ended
September 30,		Nine Months Ended
September 30,
2021202020212020
Weighted average Black-Scholes fair value of stock options granted$203.51$102.41$156.86$78.78
Risk-free interest rate
0.6%-0.9%
0.2%-0.4%
0.5%-1.1%
0.2%-1.5%
Dividend yield—%—%—%—%
Volatility
126.4%-140.0%
135.4%-152.2%
124.7%-142.0%
116.0%-152.2%
Expected term (in years)
4.1-6.1
4.0-5.3
4.1-6.1
4.0-7.6