Collateral Positions (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Collateral Positions
Assets Pledged	$ 10,970,161,000	$ 10,624,276,000
Collateral Held	408,118,000	408,833,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	10,908,025,000	10,550,345,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,682,380,000	6,678,384,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	3,817,527,000	3,463,128,000
Non-Agency MBS acquired from consolidated VIEs	1,966,000,000	1,860,000,000
Non-Agency MBS pledged as collateral in connection with contemporaneous repurchase and reverse repurchase agreements entered into with a single counterparty	761,700,000	759,200,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	408,118,000	408,833,000
Reverse Repurchase Agreements:
Collateral Positions
Collateral Held	408,118,000	408,833,000
Reverse Repurchase Agreements: | U.S. Treasury securities
Collateral Positions
Collateral Held	408,118,000	408,833,000
Derivative contracts
Collateral Positions
Assets Pledged	62,136,000	73,931,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	57,119,000	68,915,000
Derivative contracts | Cash
Collateral Positions
Assets Pledged	$ 5,017,000	$ 5,016,000