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Employee Benefit Plans Summary of all Directors' Restricted Stock Activity (Detail) - Non Employee Director Restricted Equity Awards - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Number of Shares      
Beginning of year 55,839 89,427 81,579
Add (deduct):      
Granted 53,284 55,839 89,427
Vested (55,839) (89,427) (81,579)
End of year 53,284 55,839 89,427
Weighted Average Grant Date Fair Value      
Beginning of year $ 33.24 $ 27.23 $ 25.45
Granted 40.54 33.24 27.23
Vested 33.24 27.23 25.45
End of year $ 40.54 $ 33.24 $ 27.23