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v3.21.2

Subsequent Events (Narrative) (Details) - Agency eligible investor loans - Settled Whole Loans - USD ($) $ in Thousands | Nov. 05, 2021 | Sep. 30, 2021
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Subsequent Event [Line Items] |  |
Securitization of eligible investor loans |  | $ 963,462
Subsequent Event |  |
Subsequent Event [Line Items] |  |
Securitization of eligible investor loans | $ 312,300 |

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- Definition Purchased Performing Loans + References No definition available. + Details
Name: | mfa_PurchasedPerformingLoans
Namespace Prefix: | mfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mfa_SettledWholeLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |