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Employee Benefits - Status of Service Based Restricted Shares (Detail) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Service Based Restricted Stock [Member]      
Number of Shares      
Nonvested, Beginning Balance 210,538    
Canceled (7,345)    
Vested (136,895)    
Nonvested, Ending Balance 66,298 210,538  
Weighted Average Grant Date Fair Value      
Nonvested, Beginning Balance $ 66.65    
Canceled 72.11    
Vested 63.30    
Nonvested, Ending Balance $ 72.98 $ 66.65  
Service Based Restricted Stock [Member] | Omnibus Incentive Compensation Plan (OICP) [Member]      
Number of Shares      
Granted 0    
Canceled 0    
Vested 0 0 0
Nonvested, Ending Balance 0    
Service Based Restricted Stock Unit [Member] | Omnibus Incentive Compensation Plan (OICP) [Member]      
Number of Shares      
Nonvested, Beginning Balance 179,278    
Granted 169,671    
Canceled (22,930)    
Vested (6,221)    
Nonvested, Ending Balance 319,798 179,278  
Service Based Restricted Stock Units [Member] | Omnibus Incentive Compensation Plan (OICP) [Member]      
Weighted Average Grant Date Fair Value      
Nonvested, Beginning Balance $ 66.03    
Granted 66.70    
Canceled 66.38    
Vested 71.76    
Nonvested, Ending Balance $ 66.25 $ 66.03