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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Options Activity The following table summarizes our option activity as of December 31, 2020 and 2019:
Number of
Options
Weighted
Average
Exercise
Price per
Share
Weighted
Average
Grant
Date Fair
Value per
Share
Weighted-
Average
Remaining
Contractual
Term
Aggregate
Intrinsic
Value (1)
(in thousands)
Outstanding at December 31, 201945,536,915 $13.82 $7.35 7.2 years$286,310 
Granted
5,036,520 36.04 19.30 
Exercised
(13,888,434)12.93 6.68 
Canceled/forfeited
(2,627,468)18.14 10.56 
Outstanding at December 31, 202034,057,533 17.14 9.12 6.7 years2,976,235 
Exercisable at December 31, 202017,461,374 10.99 5.67 5.4 years1,632,365 
Expected to vest at December 31, 202016,596,159 23.61 12.75 8.2 years1,343,870 
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(1)Aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of common stock for those options in the money as of December 31, 2020.
Restricted Common Stock Units Activity The following table summarizes our restricted stock unit activity during the year ended December 31, 2020:
Number of Units
Weighted Average
Grant Date Price
per Unit
Outstanding, non-vested at December 31, 20191,177,249 $19.01 
Issued
1,478,093 37.37 
Vested
(247,349)20.28 
Canceled/forfeited
(215,033)23.64 
Outstanding, non-vested at December 31, 20202,192,960 30.85 
Weighted-Average Assumptions Used to Estimate the Fair Value of Options Granted The weighted-average assumptions used to estimate the fair value of options granted and ESPP for the years ended December 31, 2020, 2019 and 2018 are as follows:
Weighted Average
Years Ended December 31,
202020192018
Options:
    Risk-free interest rate
0.83 %2.29 %2.76 %
    Expected term
6.11 years6.07 years6.27 years
    Expected volatility
58 %61 %63 %
    Expected dividends
— %— %— %
    Weighted average fair value per share
$19.30 $11.35 $9.33 
ESPP:
Risk-free interest rate
0.14 %1.95 %*
Expected term
0.50 years0.50 years*
Expected volatility
54 %53 %*
Expected dividends
— %— %*
Weighted average fair value per share
$32.18 $5.98 *
_________
* - Not applicable
Stock-Based Compensation Expense
The following table presents the components and classification of stock-based compensation expense for the years ended December 31, 2020, 2019 and 2018 as follows (in thousands):
Years Ended December 31,
202020192018
Options
$77,748 $74,780 $63,288 
Restricted common stock and units
12,401 5,125 9,277 
Employee stock purchase plan
2,872 1,217 — 
Total
$93,021 $81,122 $72,565 
Research and development
$56,155 $48,259 $37,659 
Selling, general and administrative36,866 32,863 34,906 
Total
$93,021 $81,122 $72,565