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Stock-Based Compensation - Schedule of Weighted-Average Grant-Date Fair Value of Unvested Restricted Stock (Detail) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Executive Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Grant-date fair value per share, beginning of year (in dollars per share) $ 50.69    
Restriction lapses (in dollars per share) (50.69)    
Grant-date fair value per share, end of year (in dollars per share)   $ 50.69  
Executive Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Grant-date fair value per share, beginning of year (in dollars per share) 67.06    
Grants (in dollars per share) 82.56    
Estimated additional performance shares (in dollars per share) 60.31    
Restriction lapses (in dollars per share) (50.64)    
Grant-date fair value per share, end of year (in dollars per share) 75.05 67.06  
Directors Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Grants (in dollars per share) 76.37 $ 88.19 $ 73.92
Restriction lapses (in dollars per share) (76.37)    
Directors Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Grants (in dollars per share) 76.37    
Restriction lapses (in dollars per share) $ (76.37)