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Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units and Performance Stock Units - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Options Outstanding      
Unvested at December 31, 2020 665,540    
Awarded 164,968    
Vested (185,922)    
Forfeited/expired/cancelled (24,825)    
Unvested at September 31, 2021 619,761 665,540 675,567
Unvested and expected to vest at September 30, 2021 [1] 623,642    
Weighted-Average Remaining Contractual Term (in years)      
Unvested at December 31, 2020 2 years 11 months 8 days 3 years 3 months 25 days  
Unvested and expected to vest at June 30, 2021 [1] 2 years 7 months 9 days    
Aggregate Intrinsic Value      
Unvested at September 31, 2021 $ 179,105 $ 127,904  
Unvested and expected to vest at September 30, 2021(1) [1] $ 180,226    
[1] Represents the number of vested options as of September 30, 2021 plus the number of unvested options expected to vest as of September 30, 2021 based on the unvested outstanding options at September 30, 2021 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.