Repurchase Agreements Schedule of Repurchase Agreements by Collateral Type (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Assets Sold under Agreements to Repurchase [Line Items]
Due to Correspondent Brokers	$ 413,086	$ 45,565
Assets Sold under Agreements to Repurchase, Repurchase Liability	8,693,756	6,660,148
Assets Sold under Agreements to Repurchase, Weighted Average Borrowing Rate, Excluding U.S. Treasuries and Interest Rate Swaps	0.69%	0.78%
Assets Sold under Agreements to Repurchase, Weighted Average Remaining Maturity, Excluding U.S. Treasuries and Interest Rate Swaps	80	73
Securities Sold under Agreements to Repurchase	8,693,756	6,660,148
Assets Sold under Agreements to Repurchase, Interest Rate	0.63%	0.68%
US Treasury Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	1,001,250	1,001,250
Assets Sold under Agreements to Repurchase, Interest Rate	0.20%	0.12%
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	6,499,814	4,804,533
Assets Sold under Agreements to Repurchase, Interest Rate	0.43%	0.50%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	1,009,353	731,014
Assets Sold under Agreements to Repurchase, Interest Rate	2.29%	2.61%
Inverse Interest-Only Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	178,048	118,032
Assets Sold under Agreements to Repurchase, Interest Rate	1.14%	0.97%
Loans Held-for-sale, Mortgages [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 5,291	$ 5,319
Assets Sold under Agreements to Repurchase, Interest Rate	3.14%	3.20%