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Investment Securities - Narrative (Details) | Dec. 31, 2020
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Investment securities: |
Debt Securities, Available-for-sale [Line Items] |
Average percent of amortized cost for securities in unrealized loss positions for more than 12 months | 99.20%

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- Definition Available-for-sale securities in a loss position for more than 12 months, qualitative disclosure, percent of amortized cost + References No definition available. + Details
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Namespace Prefix: | agm_
Data Type: | num:percentItemType
Balance Type: | na
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |