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Summary of Non-Vested Restricted Stock Activity (Detail) - Non-Vested Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares, Non-vested beginning balance 3,191,705    
Shares, Granted 184,265    
Shares, Vested (37,274)    
Shares, Forfeited (3,589)    
Shares, Non-vested ending balance 3,335,107 3,191,705  
Weighted-Average Grant Date Fair Value, Non-vested beginning balance $ 68.63    
Weighted-Average Grant Date Fair Value, Granted 193.89 $ 151.58 $ 167.06
Weighted-Average Grant Date Fair Value, Vested 108.36    
Weighted-Average Grant Date Fair Value, Forfeited 164.94    
Weighted-Average Grant Date Fair Value, Non-vested ending balance $ 75.00 $ 68.63