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Benefit Plans - Status of Unvested Stock Awards (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 [Roll Forward]      
Restricted stock awards, outstanding at beginning of year (in shares) 668,826 651,099 660,783
Granted (in shares) 429,122 291,034 296,411
Forfeited (in shares) (59,938) (46,914) (56,296)
Vested (in shares) (252,829) (226,393) (249,799)
Restricted stock awards, outstanding at the end of year (in shares) 785,181 668,826 651,099