Borrowed Funds (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Securities sold under agreements to repurchase:
FHLB, Principal	$ 800,000	$ 800,000
FHLB, Weighted average rate	4.53%	4.53%
Other brokers, Principal	6,150,000	6,150,000
Other brokers, Weighted average rate	4.44%	4.44%
Total securities sold under agreements to repurchase, Principal	6,950,000	6,950,000
Total securities sold under agreements to repurchase, Weighted average rate	4.45%	4.45%
Advances from the FHLB, Principal	7,225,000	8,125,000
Advances from the FHLB, Weighted average rate	3.68%	3.39%
Total borrowed funds	14,175,000	15,075,000
Total borrowed funds, Weighted average rate	4.06%	3.87%
Accrued interest payable, Principal	$ 66,565	$ 66,252