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Benefit Plans - Changes in Director's Restricted Stock Awards (Detail) - Director's Restricted Stock - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in Director's Restricted Stock Awards      
Outstanding at beginning of year (in shares) 55,510 80,117 98,086
Vested (in shares) (37,625) (24,607) (17,969)
Outstanding at end of year (in shares) 17,885 55,510 80,117