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Stock-Based Compensation - Summary of Stock Awards Outstanding (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted Stock Units (RSUs)      
Nonvested RSUs/PSUs      
Balance at period start (in shares) 666,172    
Granted (in shares) 178,190    
Vested (in shares) (204,665) (166,197) (160,200)
Forfeited (in shares) (39,457)    
Balance at period end (in shares) 600,240 666,172  
Weighted- Average Grant-Date Fair Value      
Balance at period start (in dollars per share) $ 127.77    
Granted (in dollars per share) 210.11    
Vested (in dollars per share) 130.87    
Forfeited (in dollars per share) 171.33    
Balance at period end (in dollars per share) $ 148.29 $ 127.77  
Performance Share Units (PSUs)      
Nonvested RSUs/PSUs      
Balance at period start (in shares) 456,510    
Granted (in shares) 78,830    
Vested (in shares) (235,935) (331,099) (154,189)
Unearned (in shares) (33,705)    
Forfeited (in shares) (25,600)    
Balance at period end (in shares) 240,100 456,510  
Weighted- Average Grant-Date Fair Value      
Balance at period start (in dollars per share) $ 114.04    
Granted (in dollars per share) 212.66    
Earned (in dollars per share) 89.70    
Unearned (in dollars per share) 58.77    
Forfeited (in dollars per share) 177.41    
Balance at period end (in dollars per share) $ 171.34 $ 114.04