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Borrowed Funds - Debt Disclosure by Year (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Disclosure [Abstract]    
Securities sold under repurchase agreements, maximum balance $ 210,702,000 $ 283,233,000
FHLB line of credit, maximum balance 472,000,000 173,000,000
FHLB advances, maximum balance 1,288,448,000 1,343,095,000
Securities sold under repurchase agreements, average balance 164,982,000 224,421,000
FHLB line of credit, average balance 179,003,000 37,608,000
FHLB advances, average balance $ 1,237,979,000 $ 1,315,278,000
Securities sold under repurchase agreements, weighted average interest rate 1.26% 1.42%
FHLB line of credit, weighted average interest rate 1.17% 0.61%
FHLB advances, weighted average interest rate 1.78% 1.76%