Short and Long-term Funding (Details) (USD $)	0 Months Ended
	Sep. 30, 2008	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2011
Short-term Funding
Federal funds purchased		$ 108,935,000	$ 71,385,000
Securities sold under agreements to repurchase		621,920,000	679,070,000
Federal funds purchased and securities sold under agreements to repurchase		730,855,000	750,455,000
Federal Home Loan Bank ("FHLB") advances		975,000,000	1,525,000,000
Commercial paper		63,697,000	51,484,000
Other short-term funding		1,038,697,000	1,576,484,000
Total short-term funding		1,769,552,000	2,326,939,000
Long-term Funding
Federal Home Loan Bank advances		300,364,000	400,375,000
Senior notes , at par		585,000,000	585,000,000
Subordinated debt, at par		25,821,000	25,821,000
Other long-term funding and capitalized costs		3,878,000	4,150,000
Total long-term funding		915,063,000	1,015,346,000
Total short and long-term funding		2,684,615,000	3,342,285,000
Long-term Funding (Textuals) [Abstract]
Weighted average interest rate of Long-term advances		1.80%	1.79%
Fixed coupon interest rate of subordinated debt	9.25%
New senior debt issued				155,000,000	130,000,000	300,000,000
Subordinated Debt [Member]
Long Term Debt Disclosure [Line Items]
Subordinated debentures issued	26,000,000
Senior Notes [Member]
Long Term Debt Disclosure [Line Items]
Percentage of interest rate on senior notes				1.875%	5.125%	5.125%