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Commitments and Contingencies (Residential Whole Loan Purchase Commitments) (Details) $ in Millions | Dec. 31, 2018 USD ($)
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Other Commitments [Line Items] |
Excluded amount, unsettled purchases, residential whole loans, at fair value | $ 194.7
Residential whole loans |
Other Commitments [Line Items] |
Excluded amount, unsettled purchases, residential whole loans, at fair value | $ 194.7

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- Definition Residential Whole Loans Held at Fair Value Excluded, Purchase Transaction Not Occurred + References No definition available. + Details
Name: | mfa_ResidentialWholeLoansHeldatFairValueExcludedPurchaseTransactionNotOccurred
Namespace Prefix: | mfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_OtherCommitmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancialInstrumentAxis=mfa_ResidentialWholeLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |