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Collateral Positions (Collateral Pledged and Held) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Transfers and Servicing [Abstract] |  |
Fair value pledged as collateral | $ 4,100.0 | $ 11,300.0
Accrued interest on pledged assets | $ 24.6 | $ 57.2

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- Definition Sum of the fair values as of the date of the latest financial statement presented of securities pledged as collateral and the amount of interest receivable on the securities pledged as collateral. + References No definition available. + Details
Name: | mfa_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledgedAggregateFairValueAndAccruedInterest
Namespace Prefix: | mfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728 + Details
Name: | us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_TransfersAndServicingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration