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Stock Compensation (Tables)
 12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Activity Relating to the Company's Options to Purchase Common Stock
The following table summarizes the activity relating to the Company’s options to purchase common stock for the year ended December 31, 2020:
Weighted-Average
Weighted-AverageRemaining
Number ofExercise PriceContractualAggregate
OptionsPer ShareTerm (Years)Intrinsic Value
(in thousands)
Outstanding at December 31, 20194,677,929 $39.31 6.82$109,054 
Granted234,940 $61.14 
Forfeited or cancelled (vested and unvested)(436,916)$53.95 
Exercised(299,435)$22.15 
Outstanding at December 31, 20204,176,518 $40.24 5.28$18,415 
Summary of Information on the Company's Stock Option Plan
The following table summarizes information about the Company’s stock option plan as of December 31, 2020:
Weighted-Average
Weighted-AverageRemaining
Number ofExercise PriceContractualAggregate
OptionsPer ShareTerm (Years)Intrinsic Value
(in thousands)
Vested and expected to vest at December 31, 20204,176,518 $40.24 5.28$18,415 
Exercisable at December 31, 20203,356,739 $36.96 4.62$18,154 
Schedule of Weighted-Average Assumptions Used to Compute the Stock-Based Compensation Costs for the Stock Options Granted to Employees
The following table shows the weighted-average assumptions used to compute the stock-based compensation costs for the stock options granted to employees during each of the three years ending December 31, 2020, using the Black-Scholes option-pricing model:
Year ended
December 31,
202020192018
Risk-free interest rate1.70 %2.10 %2.75 %
Dividend yield— — — 
Weighted-average expected life of options (years)6.256.256.21
Volatility81 %73 %72 %
Summary of Activity Relating to the Company's RSUs
The following table summarizes the activity relating to the Company’s RSUs for the year ended December 31, 2020:
Number ofWeighted-Average
RSUsFair Value Per Share
Outstanding and unvested at December 31, 2019245,966 $44.45 
Granted381,396 $49.14 
Forfeited or expired(113,108)$47.15 
Vested(113,020)$48.80 
Outstanding and unvested at December 31, 2020401,234 $46.92