Collateral Positions (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Collateral Positions
Assets Pledged	$ 10,635,546,000	$ 9,056,087,000
Collateral Held	509,704,000	306,401,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	10,544,383,000	8,922,898,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,818,804,000	6,549,276,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	3,215,875,000	2,067,221,000
Non-Agency MBS pledged as collateral in connection with contemporaneous repurchase and reverse repurchase agreements entered into with a single counterparty	797,300,000	448,400,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	509,704,000	306,401,000
Reverse Repurchase Agreements:
Collateral Positions
Collateral Held	509,704,000	306,401,000
Reverse Repurchase Agreements: | U.S. Treasury securities
Collateral Positions
Collateral Held	509,704,000	306,401,000
Derivative contracts
Collateral Positions
Assets Pledged	91,163,000	133,189,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	84,150,000	117,687,000
Derivative contracts | Cash
Collateral Positions
Assets Pledged	$ 7,013,000	$ 15,502,000