Borrowed Funds - Schedule of Short-Term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total short-term borrowings	$ 748,628	$ 1,080,960
Short-term borrowings
Debt Instrument [Line Items]
Securities sold under agreements to repurchase	321,628	298,960
FHLB advances	427,000	782,000
Total short-term borrowings	$ 748,628	$ 1,080,960