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Commitments and Contingencies (Corporate Loans) (Details) - Corporate Loans $ in Millions | Dec. 31, 2020 USD ($)
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Other Commitments [Line Items] |
Commitment to lend | $ 32.6
Amount drawn | $ 0.0

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- Definition Financing Receivable, Amount Drawn + References No definition available. + Details
Name: | mfa_FinancingReceivableAmountDrawn
Namespace Prefix: | mfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Financing Receivable, Commitment to Lend + References No definition available. + Details
Name: | mfa_FinancingReceivableCommitmenttoLend
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_CorporateLoans2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |