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Commitments and Contingencies (Corporate Loans) (Details) - Corporate Loan $ in Millions | Sep. 30, 2020 USD ($)
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Other Commitments [Line Items] |
Commitment to lend | $ 32.6
Remaining borrowing capacity | $ 14.5

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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 Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_CorporateLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |