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Stock-Based Compensation - Analysis of Unvested Restricted Stock (Detail) - shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 716,542 764,629 599,362
Grants (in shares) 161,909 171,974 177,493
Additional performance shares (65,473) 118,812 179,415
Restriction lapses (in shares) (282,552) (338,873) (191,641)
Forfeitures (in shares) (11,450) 0 0
Ending Balance (in shares) 518,976 716,542 764,629
Executive Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 0 12,000 35,250
Grants (in shares) 0 0 0
Additional performance shares 0 0 0
Restriction lapses (in shares) 0 (12,000) (23,250)
Forfeitures (in shares) 0 0 0
Ending Balance (in shares) 0 0 12,000
Executive Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 716,542 752,629 564,112
Grants (in shares) 151,200 156,500 159,000
Additional performance shares (65,473) 118,812 179,415
Restriction lapses (in shares) (271,843) (311,399) (149,898)
Forfeitures (in shares) (11,450) 0 0
Ending Balance (in shares) 518,976 716,542 752,629
Directors Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 0 0 0
Grants (in shares) 4,548 8,840 10,805
Additional performance shares 0 0 0
Restriction lapses (in shares) (4,548) (8,840) (10,805)
Forfeitures (in shares) 0 0 0
Ending Balance (in shares) 0 0 0
Directors Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares      
Beginning Balance (in shares) 0 0 0
Grants (in shares) 6,161 6,634 7,688
Additional performance shares 0 0 0
Restriction lapses (in shares) (6,161) (6,634) (7,688)
Forfeitures (in shares) 0 0 0
Ending Balance (in shares) 0 0 0