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Federal Funds Purchased and Securities Sold Under Agreement to Repurchase - Daily Securities Sold under Agreement to Repurchase (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Amount Outstanding at End of Period $ 29,100  
Daily Securities Sold Under Agreement to Repurchase [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Amount Outstanding at End of Period $ 6,145 $ 32,814
Weighted Average Rate End of Period 1.19% 0.20%
Maximum Amount Borrowings Outstanding Month End $ 14,929 $ 43,929
Approximate Average Outstanding in Period $ 10,253 $ 39,928
Approximate Weighted Average Interest Rate For the Period 0.96% 0.10%
Term CDs Sold Under Agreement to Repurchase [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Amount Outstanding at End of Period $ 22,925 $ 20,510
Weighted Average Rate End of Period 1.97% 1.96%
Maximum Amount Borrowings Outstanding Month End $ 22,925 $ 20,510
Approximate Average Outstanding in Period $ 21,172 $ 19,562
Approximate Weighted Average Interest Rate For the Period 1.81% 1.88%