Borrowed Funds - Schedule of Long-Term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total long-term borrowings	$ 2,315,819	$ 1,433,906
Unamortized prepayment penalties and other purchase accounting adjustments	14,300	18,300
Deferred issuance costs	1,700	1,900
Long-term borrowings
Debt Instrument [Line Items]
FHLB advances, net	1,980,666	1,031,666
Securities sold under agreements to repurchase	100,000	165,000
Subordinated debt, net	235,153	236,731
Other	0	509
Total long-term borrowings	$ 2,315,819	$ 1,433,906