Name:	fnm_FairValueSecuritiesRetainedUnconsolidatedPortfolioSecuritizationTrusts
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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481326/860-20-50-4

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

Amount, before counterparty netting, of cash proceeds received in transfers of derecognized financial assets from transactions which comprise an initial transfer and an agreement entered into in contemplation of the initial transfer resulting in retention of substantially all of the exposure to the economic return throughout the term of the transaction.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (b)
-SubTopic 20
-Topic 860
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481326/860-20-50-4D