Repurchase Agreements (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Repurchase Agreements
Balance of repurchase agreements	$ 8,568,171,000	[1]	$ 8,752,472,000	[1]
Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Weighted average remaining term-to-interest rate reset of borrowings under repurchase agreements	28 days		28 days
Effective repricing period	24 months		5 months
Balance of repurchase agreements	8,568,171,000	[2],[3]	8,752,472,000	[2]
Weighted Average Interest Rate (as a percent)	0.76%		0.85%
Overnight | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	0	[3]
Weighted Average Interest Rate (as a percent)	0.00%
Within 30 days | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	6,964,976,000	[3]
Weighted Average Interest Rate (as a percent)	0.64%
30 days to 90 days | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	1,368,088,000	[3]
Weighted Average Interest Rate (as a percent)	1.19%
Over 90 days to 12 months | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	235,107,000	[3]
Weighted Average Interest Rate (as a percent)	1.93%
Within 30 days: | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	5,841,947,000	[2]	6,293,802,000	[2]
Weighted Average Interest Rate (as a percent)	0.59%		0.71%
Over 30 days to 3 months | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	1,245,570,000	[2]	2,458,670,000	[2]
Weighted Average Interest Rate (as a percent)	1.21%		1.21%
Over 6 months to 12 months. | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	1,064,122,000	[2]	0	[2]
Weighted Average Interest Rate (as a percent)	0.69%		0.00%
Interest Rate Reset Over 12 Months [Member] | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	416,532,000	[2]	0	[2]
Weighted Average Interest Rate (as a percent)	2.10%		0.00%
Agency Mortgage Backed Securities [Member] | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	5,895,943,000		6,353,489,000
Fair Value of securities pledged as collateral under repurchase agreements	6,215,474,000		6,678,384,000
Weighted average haircut (as a percent)	4.62%	[4]	4.80%	[4]
Non Agency Mortgage Backed Securities [Member] | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	2,425,350,000	[5]	1,988,172,000	[5]
Fair Value of securities pledged as collateral under repurchase agreements	3,721,195,000	[5],[6]	3,463,128,000	[5],[6]
Weighted average haircut (as a percent)	31.91%	[4]	30.49%	[4]
Non-Agency MBS acquired from consolidated VIEs	1,883,000,000		1,860,000,000
Non Agency Mortgage Backed Securities [Member] | Linked Transactions
Repurchase Agreements
Linked repurchase agreements	82,400,000		35,300,000
U.S. Treasuries | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Balance of repurchase agreements	246,878,000		410,811,000
Fair Value of securities pledged as collateral under repurchase agreements	252,796,000		408,833,000
Weighted average haircut (as a percent)	2.00%	[4]	1.74%	[4]
Non Agency Mortgage Backed Securities [Member] | Securities Sold under Agreements to Repurchase [Member]
Repurchase Agreements
Non-Agency MBS acquired from consolidated VIEs	$ 1,883,000,000		$ 1,860,000,000

[1] The fair value of securities pledged against the Company’s repurchase agreements was $10.189 billion and $10.550 billion at September 30, 2013 and December 31, 2012, respectively.

[2] At September 30, 2013 and December 31, 2012, the Company had repurchase agreements of $82.4 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 September 30, 2013, the Company had repurchase agreements of $82.4 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 $1.883 billion and $1.860 billion of Non-Agency MBS acquired from consolidated VIEs at September 30, 2013, and December 31, 2012, respectively, that are eliminated from the Company’s consolidated balance sheets.