Short and Long-term Funding (Details) (USD $)	0 Months Ended	3 Months Ended
	Sep. 30, 2008	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Mar. 31, 2011
Short-term Funding
Federal funds purchased		$ 89,020,000	$ 154,730,000
Securities sold under agreements to repurchase		1,049,007,000	1,359,755,000
Federal Home Loan Bank ("FHLB") advances		600,000,000	1,000,000,000
Commercial paper		15,258,000	0
Total short-term funding		1,753,285,000	2,514,485,000
Long-term Funding
Federal Home Loan Bank advances		500,387,000	500,476,000
Senior notes , at par		585,000,000	430,000,000
Subordinated debt, at par		25,821,000	25,821,000
Junior subordinated debentures, at par	26,000,000	185,567,000	211,340,000
Other long-term funding and capitalized costs		8,647,000	9,434,000
Total long-term funding		1,305,422,000	1,177,071,000
Total short and long-term funding		3,058,707,000	3,691,556,000
Long Term Debt Disclosure [Line Items]
Retirement of Subordinated debt		26,000,000
Percentage of interest rate on senior notes		1.875%
Long-term Funding (Textuals) [Abstract]
Weighted average interest rate of Long-term advances		1.79%
Fixed coupon interest rate of subordinated debt	9.25%
New senior debt issued		$ 155,000,000		$ 130,000,000	$ 300,000,000
Senior Notes [Member]
Long Term Debt Disclosure [Line Items]
Percentage of interest rate on senior notes		5.125%