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Employee Benefit Plans - Activity of Restricted Stock Awards (Detail) - Restricted Stock Awards [Member] - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Number of Shares, Beginning of period 38,995 33,390 31,890
Number of Shares, Granted 16,150 16,000 13,250
Number of Shares, Vested (11,222) (10,295) (10,970)
Number of Shares, Forfeited (773) (100) (780)
Number of Shares, Nonvested, end of period 43,150 38,995 33,390
Weighted average fair value per award, Beginning of period $ 24.92 $ 22.77 $ 20.38
Weighted average fair value per award, Granted 32.02 26.35 25.50
Weighted average fair value per award, Vested 31.72 26.33 25.39
Weighted average fair value per award, Forfeited 31.54 26.07 24.97
Weighted average fair value per award, Nonvested, end of period $ 28.20 $ 24.92 $ 22.77