Repurchase Agreements (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Repurchase Agreements
Balance of repurchase agreements	$ 8,902,827,000	$ 8,752,472,000
Repurchase Agreement Borrowings
Repurchase Agreements
Weighted average remaining term-to-interest rate reset of borrowings under repurchase agreements	32 days	28 days
Effective repricing period	5 months	5 months
Balance of repurchase agreements	8,902,827,000	8,752,472,000
Weighted Average Interest Rate (as a percent)	0.80%	0.85%
Within 30 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	6,357,923,000
Weighted Average Interest Rate (as a percent)	0.65%
30 days to 90 days | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	1,952,583,000
Weighted Average Interest Rate (as a percent)	0.78%
Over 90 days to 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	216,049,000
Weighted Average Interest Rate (as a percent)	2.25%
Over 12 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	376,272,000
Weighted Average Interest Rate (as a percent)	2.62%
Within 30 days: | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	6,776,242,000	6,293,802,000
Weighted Average Interest Rate (as a percent)	0.77%	0.71%
Over 30 days to 3 months | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	2,000,536,000	2,458,670,000
Weighted Average Interest Rate (as a percent)	0.82%	1.21%
Over 6 months to 12 months. | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	126,049,000
Weighted Average Interest Rate (as a percent)	2.14%
Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	6,338,378,000	6,353,489,000
Fair Value of securities pledged as collateral under repurchase agreements	6,682,380,000	6,678,384,000
Weighted average haircut (as a percent)	4.76%	4.80%
Non-Agency MBS | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	2,155,812,000	1,988,172,000
Non-Agency MBS acquired from consolidated VIEs	1,966,000,000	1,860,000,000
Fair Value of securities pledged as collateral under repurchase agreements	3,817,527,000	3,463,128,000
Weighted average haircut (as a percent)	30.16%	30.49%
Non-Agency MBS | Linked Transactions
Repurchase Agreements
Linked repurchase agreements	34,100,000	35,300,000
U.S. Treasuries | Repurchase Agreement Borrowings
Repurchase Agreements
Balance of repurchase agreements	408,637,000	410,811,000
Fair Value of securities pledged as collateral under repurchase agreements	$ 408,118,000	$ 408,833,000
Weighted average haircut (as a percent)	0.89%	1.74%