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Stock-Based Compensation Plans - Restricted Stock Granted (Detail) - Time Based Restricted Stock [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock grants 339,196 387,841 500,943
Restricted stock grants, Weighted average grant date fair value $ 52.07 $ 38.02 $ 47.34
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock vesting period 1 year 1 year 1 year
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock vesting period 5 years 5 years 5 years
Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock grants 312,855 355,579 463,818
Outside Directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock grants 26,341 32,262 37,125