Short-Term Borrowings - Summary of Short-Term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Securities sold under repurchase agreements	$ 256,017	$ 313,062
Federal Home Loan Bank advances	2,285,000	1,025,000
Federal funds purchased	1,000,000	1,037,000
Subordinated notes	137,320	127,948
Total short-term borrowings	$ 3,678,337	$ 2,503,010