Borrowings - Summary of Short-Term Borrowings (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Debt Disclosure [Abstract]
Securities sold under repurchase agreements	$ 256,320	$ 882,696
Federal Home Loan Bank advances	450,000	820,000
Federal funds purchased	456,000	210,000
Subordinated notes	124,982	128,962
Short-term borrowings	$ 1,287,302	$ 2,041,658