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Short-term Borrowings - Interest Expense on Short-Term Borrowings (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Interest On Short Term Borrowings [Line Items]      
Total interest on short-term borrowings $ 8,799 $ 8,076 $ 5,018
Federal funds purchased      
Interest On Short Term Borrowings [Line Items]      
Total interest on short-term borrowings 77 41 54
Borrowings from FHLB      
Interest On Short Term Borrowings [Line Items]      
Total interest on short-term borrowings 8,360 7,765 4,684
Securities sold under agreements to repurchase      
Interest On Short Term Borrowings [Line Items]      
Total interest on short-term borrowings $ 362 $ 270 $ 280