Repurchase Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Repurchase Agreements
Balance of repurchase agreements	$ 8,267,388,000	$ 8,339,297,000
Repurchase agreements
Repurchase Agreements
Weighted average remaining term-to-interest rate reset of borrowings under repurchase agreements	25 days	25 days
Effective repricing period	26 months	24 months
Balance of repurchase agreements	8,267,388,000	8,339,297,000
Weighted Average Interest Rate	0.79%	0.77%
Repurchase agreements | Overnight
Repurchase Agreements
Balance of repurchase agreements	0
Weighted Average Interest Rate	0.00%
Repurchase agreements | Within 30 days
Repurchase Agreements
Balance of repurchase agreements	6,283,396,000
Weighted Average Interest Rate	0.61%
Repurchase agreements | Over 30 days to 90 days
Repurchase Agreements
Balance of repurchase agreements	1,021,076,000
Weighted Average Interest Rate	0.85%
Repurchase agreements | Over 90 days to 12 months
Repurchase Agreements
Balance of repurchase agreements	962,916,000
Weighted Average Interest Rate	1.90%
Repurchase agreements | Interest Rate Reset within 30 Days
Repurchase Agreements
Balance of repurchase agreements	7,144,737,000	7,064,598,000
Weighted Average Interest Rate	0.72%	0.68%
Repurchase agreements | Interest Rate Reset Over 30 Days to 3 Months
Repurchase Agreements
Balance of repurchase agreements	1,000,313,000	1,274,699,000
Weighted Average Interest Rate	1.12%	1.31%
Repurchase agreements | Interest Rate Reset Over 6 Months to 12 Months
Repurchase Agreements
Balance of repurchase agreements	122,338,000	0
Weighted Average Interest Rate	1.98%	0.00%
Repurchase agreements | Agency MBS
Repurchase Agreements
Balance of repurchase agreements	5,177,835,000	5,750,053,000
Fair Value of assets pledged as collateral under repurchase agreements	5,462,566,000	6,068,447,000
Weighted average haircut (as a percent)	4.79%	4.89%
Repurchase agreements | Non-Agency MBS
Repurchase Agreements
Balance of repurchase agreements	2,364,155,000	2,206,586,000
Fair Value of assets pledged as collateral under repurchase agreements	3,652,000,000	3,663,523,000
Weighted average haircut (as a percent)	28.39%	32.48%
Repurchase agreements | U.S. Treasuries
Repurchase Agreements
Balance of repurchase agreements	507,114,000	382,658,000
Fair Value of assets pledged as collateral under repurchase agreements	512,105,000	383,743,000
Weighted average haircut (as a percent)	1.62%	1.65%
Repurchase agreements | CRT securities
Repurchase Agreements
Balance of repurchase agreements	75,960,000	0
Fair Value of assets pledged as collateral under repurchase agreements	94,610,000	0
Weighted average haircut (as a percent)	25.00%	0.00%
Repurchase agreements | Residential whole loans
Repurchase Agreements
Balance of repurchase agreements	142,324,000	0
Fair Value of assets pledged as collateral under repurchase agreements	212,986,000	0
Weighted average haircut (as a percent)	33.43%	0.00%
Repurchase agreements | Non-Agency MBS
Repurchase Agreements
Non-Agency MBS acquired from consolidated VIEs	1,275,000,000	1,885,000,000
Repurchase agreements | RPL/NPL MBS
Repurchase Agreements
Fair Value of assets pledged as collateral under repurchase agreements	160,700,000
Linked Transactions | Non-Agency MBS
Repurchase Agreements
Linked repurchase agreements	$ 1,520,000,000	$ 102,700,000