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Employee Benefit Plans - Activity of Restricted Stock Awards (Detail) - Restricted Stock Awards [Member] - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
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 33,390 31,890 30,670
Number of Shares, Granted 16,000 13,250 11,000
Number of Shares, Vested (10,295) (10,970) (9,160)
Number of Shares, Forfeited (100) (780) (620)
Number of Shares, Nonvested, end of period 38,995 33,390 31,890
Weighted average fair value per award, Beginning of period $ 22.77 $ 20.38 $ 19.05
Weighted average fair value per award, Granted 26.35 25.50 22.00
Weighted average fair value per award, Vested 26.33 25.39 21.55
Weighted average fair value per award, Forfeited 26.07 24.97 21.84
Weighted average fair value per award, Nonvested, end of period $ 24.92 $ 22.77 $ 20.38