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Fair Value of Financial Instruments (Assets Measured at Fair Value on a Nonrecurring Basis) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Fair Value Disclosures [Abstract] |  |
Other real estate, fair value | $ 96.8 | $ 257.7

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- Definition Other Real Estate, Fair Value + References No definition available. + Details
Name: | mfa_OtherRealEstateFairValue
Namespace Prefix: | mfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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