Fair Value of Financial Instruments-Fair Value Hierarchy (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Assets:
Available-for-sale securities	$ 12,653,700,000us-gaap_AvailableForSaleSecurities	$ 10,762,622,000us-gaap_AvailableForSaleSecurities
Securities obtained and pledged as collateral	506,861,000	512,105,000
Residential whole loans, at fair value	144,507,000	143,472,000
Swaps	1,051,000	3,136,000
Liabilities:
Swaps	92,480,000	62,198,000
Swaps, at fair value	92,480,000	62,198,000
Obligation to return securities obtained as collateral	506,861,000	512,105,000
Level 2
Components of financial instruments carried at fair value
Credit valuation adjustment to derivative liabilities	0
Credit valuation adjustment to derivative assets	0
Recurring basis
Assets:
Securities obtained and pledged as collateral	506,861,000
Swaps	1,051,000
Total assets carried at fair value	13,306,119,000
Liabilities:
Swaps	92,480,000
Obligation to return securities obtained as collateral	506,861,000
Total liabilities carried at fair value	599,341,000
Recurring basis \| Level 1
Assets:
Securities obtained and pledged as collateral	506,861,000
Swaps	0
Total assets carried at fair value	506,861,000
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	506,861,000
Total liabilities carried at fair value	506,861,000
Recurring basis \| Level 2
Assets:
Securities obtained and pledged as collateral	0
Swaps	1,051,000
Total assets carried at fair value	12,654,751,000
Liabilities:
Swaps	92,480,000
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	92,480,000
Recurring basis \| Level 3
Assets:
Securities obtained and pledged as collateral	0
Swaps	0
Total assets carried at fair value	144,507,000
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	0
Agency MBS
Assets:
Available-for-sale securities	5,671,195,000	5,904,207,000
Agency MBS \| Recurring basis
Assets:
Available-for-sale securities	5,671,195,000
Agency MBS \| Recurring basis \| Level 1
Assets:
Available-for-sale securities	0
Agency MBS \| Recurring basis \| Level 2
Assets:
Available-for-sale securities	5,671,195,000
Agency MBS \| Recurring basis \| Level 3
Assets:
Available-for-sale securities	0
Non-Agency MBS, including MBS transferred to consolidated VIEs
Assets:
Available-for-sale securities	6,855,323,000	4,755,432,000
Non-Agency MBS, including MBS transferred to consolidated VIEs \| Recurring basis
Assets:
Available-for-sale securities	6,855,323,000
Non-Agency MBS, including MBS transferred to consolidated VIEs \| Recurring basis \| Level 1
Assets:
Available-for-sale securities	0
Non-Agency MBS, including MBS transferred to consolidated VIEs \| Recurring basis \| Level 2
Assets:
Available-for-sale securities	6,855,323,000
Non-Agency MBS, including MBS transferred to consolidated VIEs \| Recurring basis \| Level 3
Assets:
Available-for-sale securities	0
CRT securities
Assets:
Available-for-sale securities	127,182,000	102,983,000
CRT securities \| Recurring basis
Assets:
Available-for-sale securities	127,182,000
CRT securities \| Recurring basis \| Level 1
Assets:
Available-for-sale securities	0
CRT securities \| Recurring basis \| Level 2
Assets:
Available-for-sale securities	127,182,000
CRT securities \| Recurring basis \| Level 3
Assets:
Available-for-sale securities	0
Residential whole loans \| Recurring basis
Assets:
Residential whole loans, at fair value	144,507,000
Residential whole loans \| Recurring basis \| Level 1
Assets:
Residential whole loans, at fair value	0
Residential whole loans \| Recurring basis \| Level 2
Assets:
Residential whole loans, at fair value	0
Residential whole loans \| Recurring basis \| Level 3
Assets:
Residential whole loans, at fair value	$ 144,507,000

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of credit risk valuation adjustment to derivative assets to properly reflect the credit quality of the counterparties.

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Fair value of financial and nonfinancial obligations.

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The fair value of collateral received by the entity associated with securities borrowed that must be returned if the counterparty fulfills its obligations. It appears on the balance sheet as a liability and has a corresponding asset that appears on the balance sheet called Securities Received as Collateral.

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