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Employee Benefits - Status of Performance Based Restricted Shares (Detail) - Performance Based Restricted Stock Unit [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of Shares      
Nonvested, Beginning Balance 30,545    
Vested (30,545)    
Nonvested, Ending Balance   30,545  
Weighted Average Grant Date Fair Value      
Nonvested, Beginning Balance $ 75.25    
Vested $ 75.25    
Nonvested, Ending Balance   $ 75.25  
Omnibus Incentive Compensation Plan (OICP) [Member]      
Number of Shares      
Nonvested, Beginning Balance 84,886    
Granted 65,093    
Canceled (1,906)    
Vested 0 0 0
Nonvested, Ending Balance 148,073 84,886  
Weighted Average Grant Date Fair Value      
Nonvested, Beginning Balance $ 67.30    
Granted 61.67    
Canceled 68.16    
Nonvested, Ending Balance $ 64.82 $ 67.30