Name:	fnm_FairValueSecuritiesRetainedUnconsolidatedPortfolioSecuritizationTrusts
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Represents the maximum amount we could recover through available credit enhancements and recourse with third parties on guarantees recognized in our on and off balance sheets. Recoverability of such credit enhancements and recourse is subject to, among other factors, our mortgage insurers' and financial guarantors' ability to meet their obligations to us.

No definition available.

Name:	fnm_MaximumRecovery
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

We are considered to be the transferor when we transfer assets from our portfolio in a portfolio securitization transaction and the following represents the unpaid principal balance of portfolio securitizations which occurred during the period.

No definition available.

Name:	fnm_UnpaidPrincipalBalanceOfPortfolioSecuritizations
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the unpaid principal balance of securities retained in unconsolidated portfolio securitization trusts as of the period end.

No definition available.

Name:	fnm_UnpaidPrincipalBalanceSecuritiesRetainedUnconsolidatedPortfolioSecuritizationTrusts
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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.

No definition available.

Name:	us-gaap_AssumptionForFairValueAsOfBalanceSheetDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Cash flows between transferee and transferor in payment of the transferor's beneficial interest related to a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction.

No definition available.

Name:	us-gaap_CashFlowsBetweenTransferorAndTransfereeBeneficialInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12069-110248

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InvestmentsByConsolidatedAndNonconsolidatedEntitiesAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_GuaranteeTypeOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_QualitativeAndQuantitativeInformationTransferorsContinuingInvolvementTransferredFinancialAssetsByTransferOrTransfereeAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
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