Fair Value of Financial Instruments - Assets and Liabilities on Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Assets:
MBS and CRT securities	$ 10,762,622,000us-gaap_AvailableForSaleSecurities
Securities obtained and pledged as collateral	512,105,000	383,743,000
Residential whole loans, at fair value	143,472,000	0
Linked Transactions	398,336,000mfa_MBSLinkedTransactionsNet	28,181,000mfa_MBSLinkedTransactionsNet
Swaps	3,136,000	13,000,000
Liabilities:
Swaps	62,198,000	28,217,000
Obligation to return securities obtained as collateral	512,105,000	383,743,000
Agency MBS
Assets:
MBS and CRT securities	5,904,207,000	6,519,221,000
Non-Agency MBS
Assets:
MBS and CRT securities	4,755,432,000	4,852,137,000
CRT securities
Assets:
MBS and CRT securities	102,983,000	0
Level 2
Components of financial instruments carried at fair value
Derivative credit risk valuation adjustment	0
Recurring basis
Assets:
Securities obtained and pledged as collateral	512,105,000
Linked Transactions	398,336,000
Swaps	3,136,000
Total assets carried at fair value	11,819,671,000
Liabilities:
Swaps	62,198,000
Obligation to return securities obtained as collateral	512,105,000
Total liabilities carried at fair value	574,303,000
Recurring basis \| Agency MBS
Assets:
MBS and CRT securities	5,904,207,000
Recurring basis \| Non-Agency MBS
Assets:
MBS and CRT securities	4,755,432,000
Recurring basis \| CRT securities
Assets:
MBS and CRT securities	102,983,000
Recurring basis \| Residential whole loans
Assets:
Residential whole loans, at fair value	143,472,000
Recurring basis \| Level 1
Assets:
Securities obtained and pledged as collateral	512,105,000
Linked Transactions	0
Swaps	0
Total assets carried at fair value	512,105,000
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	512,105,000
Total liabilities carried at fair value	512,105,000
Recurring basis \| Level 1 \| Agency MBS
Assets:
MBS and CRT securities	0
Recurring basis \| Level 1 \| Non-Agency MBS
Assets:
MBS and CRT securities	0
Recurring basis \| Level 1 \| CRT securities
Assets:
MBS and CRT securities	0
Recurring basis \| Level 1 \| Residential whole loans
Assets:
Residential whole loans, at fair value	0
Recurring basis \| Level 2
Assets:
Securities obtained and pledged as collateral	0
Linked Transactions	398,336,000
Swaps	3,136,000
Total assets carried at fair value	11,164,094,000
Liabilities:
Swaps	62,198,000
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	62,198,000
Recurring basis \| Level 2 \| Agency MBS
Assets:
MBS and CRT securities	5,904,207,000
Recurring basis \| Level 2 \| Non-Agency MBS
Assets:
MBS and CRT securities	4,755,432,000
Recurring basis \| Level 2 \| CRT securities
Assets:
MBS and CRT securities	102,983,000
Recurring basis \| Level 2 \| Residential whole loans
Assets:
Residential whole loans, at fair value	0
Recurring basis \| Level 3
Assets:
Securities obtained and pledged as collateral	0
Linked Transactions	0
Swaps	0
Total assets carried at fair value	143,472,000
Liabilities:
Swaps	0
Obligation to return securities obtained as collateral	0
Total liabilities carried at fair value	0
Recurring basis \| Level 3 \| Agency MBS
Assets:
MBS and CRT securities	0
Recurring basis \| Level 3 \| Non-Agency MBS
Assets:
MBS and CRT securities	0
Recurring basis \| Level 3 \| CRT securities
Assets:
MBS and CRT securities	0
Recurring basis \| Level 3 \| Residential whole loans
Assets:
Residential whole loans, at fair value	$ 143,472,000

The fair value of forward contracts to repurchase mortgage-backed securities.

No definition available.

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- Definition

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

+ References

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-Publisher FASB

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

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- Definition

Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

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-Publisher FASB

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- Definition

Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

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No definition available.

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Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of financial and nonfinancial obligations.

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- Definition

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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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

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