Repurchase Agreements (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Repurchase Agreements
Balance of repurchase agreements	$ 8,909,283,000	[1]	$ 8,752,472,000	[1]
Repurchase Agreement Borrowings
Repurchase Agreements
Weighted average remaining term-to-interest rate reset of borrowings under repurchase agreements	28 days		28 days
Effective repricing period	9 months		5 months
Balance of repurchase agreements	8,909,283,000	[2],[3]	8,752,472,000	[2]
Weighted Average Interest Rate (as a percent)	0.79%		0.85%
Within 30 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	7,004,532,000	[3]
Weighted Average Interest Rate (as a percent)	0.67%
30 days to 90 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	1,104,778,000	[3]
Weighted Average Interest Rate (as a percent)	0.70%
Over 90 days to 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	342,043,000	[3]
Weighted Average Interest Rate (as a percent)	1.89%
Over 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	457,930,000	[3]
Weighted Average Interest Rate (as a percent)	2.04%
Overnight | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	0	[3]
Weighted Average Interest Rate (as a percent)	0.00%
Within 30 days: | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	7,503,586,000	[2]	6,293,802,000	[2]
Weighted Average Interest Rate (as a percent)	0.76%		0.71%
Over 30 days to 3 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	1,152,390,000	[2]	2,458,670,000	[2]
Weighted Average Interest Rate (as a percent)	0.75%		1.21%
Over 6 months to 12 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	253,307,000	[2]	0	[2]
Weighted Average Interest Rate (as a percent)	1.83%		0.00%
Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	6,080,964,000		6,353,489,000
Fair Value of securities pledged as collateral under repurchase agreements	6,443,433,000		6,678,384,000
Weighted average haircut (as a percent)	4.65%	[4]	4.80%	[4]
Non-Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	2,424,349,000	[5]	1,988,172,000	[5]
Non-Agency MBS acquired from consolidated VIEs	1,935,000,000		1,860,000,000
Fair Value of securities pledged as collateral under repurchase agreements	3,906,157,000	[5],[6]	3,463,128,000	[5],[6]
Weighted average haircut (as a percent)	30.19%	[4]	30.49%	[4]
Non-Agency MBS | Linked Transactions
Repurchase Agreements
Linked repurchase agreements	33,200,000		35,300,000
U.S. Treasuries | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	403,970,000		410,811,000
Fair Value of securities pledged as collateral under repurchase agreements	$ 405,458,000		$ 408,833,000
Weighted average haircut (as a percent)	0.89%	[4]	1.74%	[4]

[1] The fair value of securities pledged against the Company’s repurchase agreements was $10.755 billion and $10.550 billion at June 30, 2013 and December 31, 2012, respectively.

[2] At June 30, 2013 and December 31, 2012, the Company had repurchase agreements of $33.2 million and $35.3 million, respectively, that were linked to Non-Agency MBS purchases and accounted for as Linked Transactions, and as such, the linked repurchase agreements are not included in the above table. (See Note 5)

[3] At June 30, 2013, the Company had repurchase agreements of $33.2 million that were linked to Non-Agency MBS purchases and were accounted for as Linked Transactions, and as such, the linked repurchase agreements are not included in the above table. (See Note 5)

[4] Haircut represents the percentage amount by which the collateral value is contractually required to exceed the loan amount on the Company’s repurchase agreements borrowings.

[5] Does not reflect Non-Agency MBS and repurchase agreement borrowings that are components of Linked Transactions.

[6] Includes $1.935 billion and $1.860 billion of Non-Agency MBS acquired from consolidated VIEs at June 30, 2013, and December 31, 2012, respectively, that are eliminated from the Company’s consolidated balance sheet.