Borrowed Funds - Additional Information (Detail) (USD $)	3 Months Ended				12 Months Ended			0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 27, 2013
Debt Instrument [Line Items]
Prepaid long-term FHLB advances	$ 150,000,000				$ 150,000,000
Prepaid long-term FHLB stock	125,000,000				125,000,000
Combined weighted average interest rate	4.52%				4.52%
Gains (Losses) on Extinguishment of Debt	(10,132,000)	0	0	0	(10,132,000)	0	0
Weighted average interest rate for short-term borrowings	0.28%				0.28%	0.28%
Interest expense recorded on FHLB advances					78,200,000	78,400,000	79,500,000
Callable FHLB advances with scheduled maturities beyond 2017	810,000,000				810,000,000
Callable FHLB advances interest rates range, low	2.27%				2.27%
Callable FHLB advances interest rates range, high	4.09%				4.09%
Net carrying value of subordinated debentures	131,300,000				131,300,000	121,800,000
Interest expense on subordinated debentures					8,500,000	5,900,000	5,000,000
Fair value of securities						1,300,000,000
Subordinated notes [Member]
Debt Instrument [Line Items]
Subordinated notes	100,000,000				100,000,000
Subordinated notes, interest rate					5.00%
Subordinated notes, due date					Jul. 15, 2015
FHLB [Member]
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings	3.83%				3.83%	3.89%
Collateralized letter of credit	350,000,000				350,000,000
Long-term borrowings [Member]
Debt Instrument [Line Items]
Securities sold under agreements to repurchase	462,500,000				462,500,000	587,500,000
Subordinated notes	231,308,000				231,308,000	221,757,000
Subordinated debentures [Member]
Debt Instrument [Line Items]
Subordinated notes								$ 125,000,000
Subordinated notes, interest rate								5.125%
Subordinated notes, due date								Sep. 27, 2023