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Summary of Significant Accounting Policies (Financing Agreements) (Details) | 12 Months Ended
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Dec. 31, 2020
Accounting Policies [Abstract] |
Repurchase financing period, low end of range | 1 month
Repurchase financing period, high end of range | 6 months
Repurchase agreements financing collateralized, period | 12 months

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- Definition Repurchase Agreements Financing Collateralized, Period + References No definition available. + Details
Name: | mfa_RepurchaseAgreementsFinancingCollateralizedPeriod
Namespace Prefix: | mfa_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the high end of the range of the typical period for repurchase financing agreements. + References No definition available. + Details
Name: | mfa_RepurchaseAgreementsFinancingPeriodHighEndOfRange
Namespace Prefix: | mfa_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the low end of the range of the typical period for repurchase financing agreements. + References No definition available. + Details
Name: | mfa_RepurchaseAgreementsFinancingPeriodLowEndOfRange
Namespace Prefix: | mfa_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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