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Share-Based Compensation - Share-Based Compensation Amounts Included in Net Income (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total compensation expense $ 1,876 $ 1,056  
Less: tax benefit (496) (275)  
Share-Based Compensation amounts included in net income $ 1,380 781  
Awards to non-employee directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 887    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 84.47    
Total compensation expense $ 188 176  
Award to key employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number 193,920   186,878
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period 66,425    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period 53,147    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expired In Period (852)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value $ 94.61   $ 87.06
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value 102.73    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value 76.31    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expirations Weighted Average Grant Date Fair Value $ 74.85    
Total compensation expense $ 1,688 $ 880