Collateral Positions (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Collateral Positions
Assets Pledged	$ 10,315,995,000		$ 10,624,276,000
Collateral Held	252,796,000		408,833,000
Derivative contracts
Collateral Positions
Assets Pledged	126,530,000		73,931,000
Derivative contracts | Agency MBS
Collateral Positions
Assets Pledged	67,255,000		68,915,000
Derivative contracts | Cash
Collateral Positions
Assets Pledged	59,275,000	[1]	5,016,000	[1]
Repurchase Agreement Borrowings
Collateral Positions
Assets Pledged	10,189,465,000		10,550,345,000
Repurchase Agreement Borrowings | Agency MBS
Collateral Positions
Assets Pledged	6,215,474,000		6,678,384,000
Repurchase Agreement Borrowings | Non-Agency MBS
Collateral Positions
Assets Pledged	3,721,195,000	[2],[3]	3,463,128,000	[2],[3]
Non-Agency MBS acquired from consolidated VIEs	1,883,000,000		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	742,900,000		759,200,000
Repurchase Agreement Borrowings | U.S. Treasury securities
Collateral Positions
Assets Pledged	252,796,000		408,833,000
Reverse Repurchase Agreements:
Collateral Positions
Collateral Held	252,796,000		408,833,000
Reverse Repurchase Agreements: | U.S. Treasury securities
Collateral Positions
Collateral Held	$ 252,796,000		$ 408,833,000

[1] Cash pledged as collateral is reported as “restricted cash” on the Company’s consolidated balance sheets.

[2] Includes $1.883 billion and $1.860 billion of Non-Agency MBS acquired in connection with resecuritization transactions from consolidated VIEs at September 30, 2013 and December 31, 2012, respectively, that are eliminated from the Company’s consolidated balance sheets.

[3] In addition, $742.9 million and $759.2 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 September 30, 2013 and December 31, 2012, respectively.