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v3.20.4

Equity Compensation, Employment Agreements and Other Benefit Plans (Savings Plan) (Details) - Savings Plan - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Employer contribution percentage on first 3 percent of eligible compensation deferred by employees |  | 100.00% |
Percentage of eligible compensation deferred by employees qualifying for 100 percent matching contribution |  | 3.00% |
Employer contribution percentage on next 2 percent of eligible compensation deferred by employees |  | 50.00% |
Percentage of eligible compensation deferred by employees qualifying for 50 percent matching contribution |  | 2.00% |
Percentage of employer matching contributions that vest immediately |  | 100.00% |
Expenses for matching contributions | $ 371,000 | $ 480,000 | $ 503,500

X
- Definition Represents the percentage of the employer's matching contribution on the first tier of eligible compensation deferred by employees. + References No definition available. + Details
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Namespace Prefix: | mfa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of the employer's matching contribution on the second tier of eligible compensation deferred by employees. + References No definition available. + Details
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Namespace Prefix: | mfa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of the first tier of eligible compensation deferred by employees and receiving the highest level of employer match. + References No definition available. + Details
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Namespace Prefix: | mfa_
Data Type: | num:percentItemType
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Period Type: | duration

X
- Definition Represents the percentage of the second tier of eligible compensation deferred by employees and receiving the second level of employer match. + References No definition available. + Details
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Namespace Prefix: | mfa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of employer matching contributions under a deferred compensation arrangement that vest immediately. + References No definition available. + Details
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Namespace Prefix: | mfa_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contributions made by employer into fund established for purposes of making future disbursement to individual in accordance with deferred compensation arrangement. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |