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Stockholders' Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2018
Stockholders Equity Note Disclosure [Line Items]            
Stock-based compensation expense $ 6,792 $ 4,225 $ 20,476 $ 12,492    
Incentive options, term     10 years      
Stock options, outstanding 658,600 723,914 658,600 723,914 696,711  
Closing price of common stock $ 288.99   $ 288.99      
Aggregate intrinsic value of stock options exercised     $ 14,500 $ 30,700    
Weighted average grant date fair value of share-based awards granted     $ 88.01 $ 48.13    
Total fair value of stock options vested     $ 2,600 $ 2,600    
Total unrecognized compensation cost $ 61,300   $ 61,300      
Unrecognized compensation cost, weighted average remaining requisite service period     3 years 29 days      
Number of unvested options and restricted stock units 1,950,908   1,950,908      
2018 Plan            
Stockholders Equity Note Disclosure [Line Items]            
Common stock shares reserved for Issuance           2,778,000
Employee Stock Option | Minimum            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting period     3 years      
Employee Stock Option | Minimum | Vest Over Three Year            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting percentage     20.00%      
Employee Stock Option | Maximum            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting period     5 years      
Employee Stock Option | Maximum | Vest Over Three Year            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting percentage     33.00%      
Non-Employee Directors            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting period     9 years      
Unvested Options            
Stockholders Equity Note Disclosure [Line Items]            
Incentive options, vesting period     5 years      
Restricted Stock Units and Performance Stock Units            
Stockholders Equity Note Disclosure [Line Items]            
Restricted stock units, outstanding 619,761 675,567 619,761 675,567 665,540  
Closing price of common stock $ 288.99   $ 288.99      
Aggregate intrinsic value of restricted stock units vested     $ 39,200 $ 25,000    
Weighted average grant date fair value of restricted stock units granted     $ 58.10 $ 43.73    
Total grant date fair value of restricted stock units vested     $ 10,800 $ 9,800