Collateral Positions (Details 2) (USD $)	Mar. 31, 2014		Dec. 31, 2013
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	$ 10,582,825,000
U.S. Treasury securities
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	435,888,000
Agency MBS
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	6,421,137,000
Agency MBS | Fannie Mae.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	5,349,917,000
Agency MBS | Freddie Mac.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	1,060,887,000
Agency MBS | Ginnie Mae.
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	10,333,000
Non-Agency MBS
Collateral Positions
Total Fair Value of MBS Assets Pledged and Accrued Interest	3,725,800,000	[1],[2]
Securities Sold under Agreements to Repurchase
Collateral Positions
Assets Pledged	10,475,720,000
Amortized Cost	9,557,048,000
Accrued Interest on Pledged MBS	29,965,000
Securities Sold under Agreements to Repurchase | U.S. Treasury securities
Collateral Positions
Assets Pledged	435,888,000
Amortized Cost	435,888,000
Accrued Interest on Pledged MBS	0
Securities Sold under Agreements to Repurchase | Agency MBS
Collateral Positions
Assets Pledged	6,327,835,000
Amortized Cost	6,296,627,000
Accrued Interest on Pledged MBS	16,162,000
Securities Sold under Agreements to Repurchase | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	5,328,922,000
Amortized Cost	5,281,673,000
Accrued Interest on Pledged MBS	13,620,000
Securities Sold under Agreements to Repurchase | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	998,913,000
Amortized Cost	1,014,954,000
Accrued Interest on Pledged MBS	2,542,000
Securities Sold under Agreements to Repurchase | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	0
Amortized Cost	0
Accrued Interest on Pledged MBS	0
Securities Sold under Agreements to Repurchase | Non-Agency MBS
Collateral Positions
Assets Pledged	3,711,997,000	[1],[2]
Amortized Cost	2,824,533,000	[1],[2]
Accrued Interest on Pledged MBS	13,803,000	[1],[2]
Non-Agency MBS acquired from consolidated VIEs in connection with resecuritization transactions	1,936,000,000		1,885,000,000
Non-Agency MBS pledged as collateral in connection with contemporaneous repurchase and reverse repurchase agreements entered into with a single counterparty	751,100,000		738,300,000
Derivative contracts
Collateral Positions
Assets Pledged	76,968,000
Amortized Cost	77,852,000
Accrued Interest on Pledged MBS	172,000
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	76,968,000
Amortized Cost	77,852,000
Accrued Interest on Pledged MBS	172,000
Derivative contracts | Agency MBS | Fannie Mae.
Collateral Positions
Assets Pledged	7,360,000
Amortized Cost	7,086,000
Accrued Interest on Pledged MBS	15,000
Derivative contracts | Agency MBS | Freddie Mac.
Collateral Positions
Assets Pledged	59,289,000
Amortized Cost	60,642,000
Accrued Interest on Pledged MBS	143,000
Derivative contracts | Agency MBS | Ginnie Mae.
Collateral Positions
Assets Pledged	10,319,000
Amortized Cost	10,124,000
Accrued Interest on Pledged MBS	14,000
Derivative contracts | Non-Agency MBS
Collateral Positions
Assets Pledged	0	[1],[2]
Amortized Cost	0	[1],[2]
Accrued Interest on Pledged MBS	$ 0	[1],[2]

[1] In addition, $751.1 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 March 31, 2014

[2] Includes $1.936 billion of Non-Agency MBS acquired in connection with resecuritization transactions from consolidated VIEs at March 31, 2014, that are eliminated from the Company’s consolidated balance sheets.