Collateral Positions (Details 2) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Collateral Positions
Assets Pledged	$ 9,056,087,000	$ 6,891,431,000
Total Fair Value of MBS Pledged and Accrued Interest	9,075,696,000
Agency MBS
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	6,688,482,000
Agency MBS | Fannie Mae.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	5,979,468,000
Agency MBS | Freddie Mac.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	695,857,000
Agency MBS | Ginnie Mae.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	13,157,000
Non-Agency MBS
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	2,080,813,000
Non-Agency MBS | Rated AAA
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	8,783,000
Non-Agency MBS | Rated A
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	22,665,000
Non-Agency MBS | Rated BBB
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	4,720,000
Non-Agency MBS | Rated BB
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	294,986,000
Non-Agency MBS | Rated B
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	80,748,000
Non-Agency MBS | Rated CCC
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	167,989,000
Non-Agency MBS | Rated CC
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	66,687,000
Non-Agency MBS | Rated C
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	227,245,000
Non-Agency MBS | D-rated
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	154,208,000
Non-Agency MBS | Not Rated
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	1,052,782,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	8,922,898,000	6,702,814,000
Amortized Cost	8,777,945,000
Accrued Interest on Pledged MBS	34,769,000
Repurchase Agreement Borrowings | U.S. Treasuries
Collateral Positions
Assets Pledged	306,401,000
Amortized Cost	306,908,000
Total Fair Value of MBS Pledged and Accrued Interest	306,401,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,549,276,000
Amortized Cost	6,345,308,000
Accrued Interest on Pledged MBS	21,177,000
Repurchase Agreement Borrowings | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	5,859,425,000
Amortized Cost	5,675,406,000
Accrued Interest on Pledged MBS	18,432,000
Repurchase Agreement Borrowings | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	686,595,000
Amortized Cost	666,693,000
Accrued Interest on Pledged MBS	2,739,000
Repurchase Agreement Borrowings | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	3,256,000
Amortized Cost	3,209,000
Accrued Interest on Pledged MBS	6,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	2,067,221,000
Amortized Cost	2,125,729,000
Accrued Interest on Pledged MBS	13,592,000
Non-Agency MBS acquired from consolidated VIEs in connection with resecuritization transactions	1,375,000,000	462,000,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated AAA
Collateral Positions
Assets Pledged	8,739,000
Amortized Cost	8,913,000
Accrued Interest on Pledged MBS	44,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated A
Collateral Positions
Assets Pledged	22,597,000
Amortized Cost	26,680,000
Accrued Interest on Pledged MBS	68,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BBB
Collateral Positions
Assets Pledged	4,693,000
Amortized Cost	5,041,000
Accrued Interest on Pledged MBS	27,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BB
Collateral Positions
Assets Pledged	293,784,000
Amortized Cost	291,862,000
Accrued Interest on Pledged MBS	1,202,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated B
Collateral Positions
Assets Pledged	80,413,000
Amortized Cost	85,795,000
Accrued Interest on Pledged MBS	335,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CCC
Collateral Positions
Assets Pledged	167,157,000
Amortized Cost	173,816,000
Accrued Interest on Pledged MBS	832,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CC
Collateral Positions
Assets Pledged	66,365,000
Amortized Cost	64,961,000
Accrued Interest on Pledged MBS	322,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated C
Collateral Positions
Assets Pledged	225,829,000
Amortized Cost	239,553,000
Accrued Interest on Pledged MBS	1,416,000
Repurchase Agreement Borrowings | Non-Agency MBS | D-rated
Collateral Positions
Assets Pledged	152,953,000
Amortized Cost	162,314,000
Accrued Interest on Pledged MBS	1,255,000
Repurchase Agreement Borrowings | Non-Agency MBS | Not Rated
Collateral Positions
Assets Pledged	1,044,691,000
Amortized Cost	1,066,794,000
Accrued Interest on Pledged MBS	8,091,000
Derivative contracts
Collateral Positions
Assets Pledged	133,189,000	188,617,000
Amortized Cost	113,694,000
Accrued Interest on Pledged MBS	342,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	117,687,000
Amortized Cost	113,694,000
Accrued Interest on Pledged MBS	342,000
Derivative contracts | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	101,316,000
Amortized Cost	97,723,000
Accrued Interest on Pledged MBS	295,000
Derivative contracts | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	6,495,000
Amortized Cost	6,321,000
Accrued Interest on Pledged MBS	28,000
Derivative contracts | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	9,876,000
Amortized Cost	9,650,000
Accrued Interest on Pledged MBS	19,000
Derivative contracts | Non-Agency MBS
Collateral Positions
Assets Pledged	117,687,000
Amortized Cost	113,694,000
Accrued Interest on Pledged MBS	$ 342,000