Collateral Positions (Details 2) (USD $)	Jun. 30, 2013		Dec. 31, 2012
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	$ 10,831,072,000
U.S. Treasury securities
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	405,458,000
Agency MBS
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	6,502,084,000
Agency MBS | Fannie Mae.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	5,305,150,000
Agency MBS | Freddie Mac.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	1,184,270,000
Agency MBS | Ginnie Mae.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	12,664,000
Non-Agency MBS
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	3,923,530,000	[1],[2],[3]
Non-Agency MBS | Rated AAA
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	18,693,000
Non-Agency MBS | Rated BBB
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	31,124,000
Non-Agency MBS | Rated BB
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	333,986,000
Non-Agency MBS | Rated B
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	152,633,000
Non-Agency MBS | Rated CCC
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	368,210,000
Non-Agency MBS | Rated CC
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	144,599,000
Non-Agency MBS | Rated C
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	94,374,000
Non-Agency MBS | D-rated
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	1,184,744,000
Non-Agency MBS | Not Rated
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	1,595,167,000
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	10,755,048,000
Amortized Cost	10,032,556,000
Accrued Interest on Pledged MBS	34,649,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	405,458,000
Amortized Cost	405,458,000
Accrued Interest on Pledged MBS	0
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,443,433,000
Amortized Cost	6,410,207,000
Accrued Interest on Pledged MBS	17,276,000
Repurchase Agreement Borrowings | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	5,261,649,000
Amortized Cost	5,214,739,000
Accrued Interest on Pledged MBS	14,091,000
Repurchase Agreement Borrowings | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	1,176,891,000
Amortized Cost	1,190,698,000
Accrued Interest on Pledged MBS	3,178,000
Repurchase Agreement Borrowings | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	4,893,000
Amortized Cost	4,770,000
Accrued Interest on Pledged MBS	7,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	3,906,157,000	[1],[2],[3]
Amortized Cost	3,216,891,000	[1],[2],[3]
Accrued Interest on Pledged MBS	17,373,000	[1],[2],[3]
Non-Agency MBS acquired from consolidated VIEs in connection with resecuritization transactions	1,935,000,000		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	740,000,000		759,200,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated AAA
Collateral Positions
Assets Pledged	18,629,000
Amortized Cost	18,184,000
Accrued Interest on Pledged MBS	64,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BBB
Collateral Positions
Assets Pledged	31,020,000
Amortized Cost	30,790,000
Accrued Interest on Pledged MBS	104,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated BB
Collateral Positions
Assets Pledged	332,903,000
Amortized Cost	298,848,000
Accrued Interest on Pledged MBS	1,083,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated B
Collateral Positions
Assets Pledged	152,221,000
Amortized Cost	135,456,000
Accrued Interest on Pledged MBS	412,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CCC
Collateral Positions
Assets Pledged	366,903,000
Amortized Cost	318,387,000
Accrued Interest on Pledged MBS	1,307,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated CC
Collateral Positions
Assets Pledged	143,955,000
Amortized Cost	130,633,000
Accrued Interest on Pledged MBS	644,000
Repurchase Agreement Borrowings | Non-Agency MBS | Rated C
Collateral Positions
Assets Pledged	94,060,000
Amortized Cost	83,385,000
Accrued Interest on Pledged MBS	314,000
Repurchase Agreement Borrowings | Non-Agency MBS | D-rated
Collateral Positions
Assets Pledged	1,178,816,000
Amortized Cost	1,057,747,000
Accrued Interest on Pledged MBS	5,928,000
Repurchase Agreement Borrowings | Non-Agency MBS | Not Rated
Collateral Positions
Assets Pledged	1,587,650,000
Amortized Cost	1,143,461,000
Accrued Interest on Pledged MBS	7,517,000
Derivative contracts
Collateral Positions
Assets Pledged	41,264,000
Amortized Cost	39,886,000
Accrued Interest on Pledged MBS	111,000
Derivative contracts | Rated AAA
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated BBB
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated BB
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated B
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated CCC
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated CC
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Rated C
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | D-rated
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Not Rated
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | U.S. Treasury securities
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	41,264,000
Amortized Cost	39,886,000
Accrued Interest on Pledged MBS	111,000
Derivative contracts | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	29,328,000
Amortized Cost	28,350,000
Accrued Interest on Pledged MBS	82,000
Derivative contracts | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	4,184,000
Amortized Cost	4,033,000
Accrued Interest on Pledged MBS	17,000
Derivative contracts | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	7,752,000
Amortized Cost	7,503,000
Accrued Interest on Pledged MBS	12,000
Derivative contracts | Non-Agency MBS
Collateral Positions
Assets Pledged	0	[1],[2],[3]
Amortized Cost	0	[1],[2],[3]
Accrued Interest on Pledged MBS	$ 0	[1],[2],[3]

[1] Reported based on the lowest rating issued by a Rating Agency, if more than one rating is issued on the security, at the date presented.

[2] Includes $1.935 billion of Non-Agency MBS acquired in connection with resecuritization transactions from consolidated VIEs at June 30, 2013, that are eliminated from the Company’s consolidated balance sheet.

[3] In addition, $740.0 million of Non-Agency MBS are pledged as collateral in connection with contemporaneous repurchase and reverse repurchase agreements entered into with a single counterparty at June 30, 2013.