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Residential Whole Loans (Purchased Credit Impaired Loans - Narrative) (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Purchased credit impaired loans |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |
Allowance for loan loss | $ 244 | $ 968 | $ 330 | $ 990

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- Definition The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=118959141&loc=d3e21338-158488 + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossesRealEstate
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_FinancingReceivableImpairedLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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