Collateral Positions (Details 2) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Collateral Positions
Assets Pledged	$ 9,920,016,000	$ 9,056,087,000
Total Fair Value of MBS Pledged and Accrued Interest	9,947,155,000
U.S. Treasury securities
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	512,907,000
Agency MBS
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	6,490,219,000
Agency MBS | Fannie Mae.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	5,813,568,000
Agency MBS | Freddie Mac.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	661,874,000
Agency MBS | Ginnie Mae.
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	14,777,000
Non-Agency MBS
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	2,944,029,000
Non-Agency MBS | Rated AAA
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	34,437,000
Non-Agency MBS | Rated A
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	20,958,000
Non-Agency MBS | Rated BBB
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	12,915,000
Non-Agency MBS | Rated BB
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	307,334,000
Non-Agency MBS | Rated B
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	90,022,000
Non-Agency MBS | Rated CCC
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	273,642,000
Non-Agency MBS | Rated CC
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	201,447,000
Non-Agency MBS | Rated C
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	297,806,000
Non-Agency MBS | D-rated
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	432,734,000
Non-Agency MBS | Not Rated
Collateral Positions
Total Fair Value of MBS Pledged and Accrued Interest	1,272,734,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	9,815,760,000	8,922,898,000
Amortized Cost	9,501,238,000
Accrued Interest on Pledged MBS	35,874,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	512,907,000
Amortized Cost	511,241,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,374,852,000
Amortized Cost	6,184,318,000
Accrued Interest on Pledged MBS	19,846,000
Repurchase Agreement Borrowings | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	5,719,487,000
Amortized Cost	5,551,799,000
Accrued Interest on Pledged MBS	17,338,000
Repurchase Agreement Borrowings | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	650,162,000
Amortized Cost	627,388,000
Accrued Interest on Pledged MBS	2,498,000
Repurchase Agreement Borrowings | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	5,203,000
Amortized Cost	5,131,000
Accrued Interest on Pledged MBS	10,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	2,928,001,000
Amortized Cost	2,805,679,000
Accrued Interest on Pledged MBS	16,028,000
Non-Agency MBS acquired from consolidated VIEs in connection with resecuritization transactions	1,610,000,000	1,375,000,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated AAA
Collateral Positions
Assets Pledged	34,322,000
Amortized Cost	34,356,000
Accrued Interest on Pledged MBS	115,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated A
Collateral Positions
Assets Pledged	20,895,000
Amortized Cost	23,493,000
Accrued Interest on Pledged MBS	63,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BBB
Collateral Positions
Assets Pledged	12,853,000
Amortized Cost	12,677,000
Accrued Interest on Pledged MBS	62,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BB
Collateral Positions
Assets Pledged	306,161,000
Amortized Cost	291,862,000
Accrued Interest on Pledged MBS	1,173,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated B
Collateral Positions
Assets Pledged	89,690,000
Amortized Cost	90,086,000
Accrued Interest on Pledged MBS	332,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CCC
Collateral Positions
Assets Pledged	272,421,000
Amortized Cost	266,026,000
Accrued Interest on Pledged MBS	1,221,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CC
Collateral Positions
Assets Pledged	200,517,000
Amortized Cost	191,788,000
Accrued Interest on Pledged MBS	930,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated C
Collateral Positions
Assets Pledged	296,214,000
Amortized Cost	294,976,000
Accrued Interest on Pledged MBS	1,592,000
Repurchase Agreement Borrowings | Non-Agency MBS | D-rated
Collateral Positions
Assets Pledged	430,125,000
Amortized Cost	429,851,000
Accrued Interest on Pledged MBS	2,609,000
Repurchase Agreement Borrowings | Non-Agency MBS | Not Rated
Collateral Positions
Assets Pledged	1,264,803,000
Amortized Cost	1,170,564,000
Accrued Interest on Pledged MBS	7,931,000
Derivative contracts
Collateral Positions
Assets Pledged	104,256,000	133,189,000
Amortized Cost	91,852,000
Accrued Interest on Pledged MBS	274,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	95,247,000
Amortized Cost	91,852,000
Accrued Interest on Pledged MBS	274,000
Derivative contracts | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	76,524,000
Amortized Cost	73,696,000
Accrued Interest on Pledged MBS	219,000
Derivative contracts | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	9,176,000
Amortized Cost	8,862,000
Accrued Interest on Pledged MBS	38,000
Derivative contracts | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	9,547,000
Amortized Cost	9,294,000
Accrued Interest on Pledged MBS	$ 17,000