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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 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
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     751    
Total compensation expense $ 609 $ 1,325 $ 1,095 $ 2,844  
Less: tax benefit (158) (363) (285) (779)  
Share-Based Compensation amounts included in net income 451 962 $ 810 2,065  
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     6,759    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period     6,759    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value     $ 93.14    
Total compensation expense $ 157 135 $ 305 269  
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 135,840   135,840   131,741
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period     45,016    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period     25,831    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expired In Period     15,086    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value $ 74.05   $ 74.05   $ 67.24
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value     91.19    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value     67.08    
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expirations Weighted Average Grant Date Fair Value     $ 69.28    
Total compensation expense $ 452 $ 1,190 $ 790 $ 2,575