Collateral Positions (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Collateral Positions
Assets Pledged	$ 9,920,016,000	$ 9,056,087,000
Collateral Held	512,907,000	306,401,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	9,815,760,000	8,922,898,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,374,852,000	6,549,276,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	2,928,001,000	2,067,221,000
Non-Agency MBS acquired from consolidated VIEs	1,610,000,000	1,375,000,000
Non-Agency MBS pledged as collateral in connection with contemporaneous repurchase and reverse repurchase agreements entered into with a single counterparty	763,400,000	448,400,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	512,907,000	306,401,000
Reverse Repurchase Agreements:
Collateral Positions
Collateral Held	512,907,000	306,401,000
Reverse Repurchase Agreements: | U.S. Treasury securities
Collateral Positions
Collateral Held	512,907,000	306,401,000
Derivative contracts
Collateral Positions
Assets Pledged	104,256,000	133,189,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	95,247,000	117,687,000
Derivative contracts | Cash
Collateral Positions
Assets Pledged	$ 9,009,000	$ 15,502,000