Name:	fnm_AllowanceForMortgageInsuranceReceivable
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Estimated benefit included in total loss reserves is the contractual mortgage insurance benefits under mortgage insurance policies net of the adjustment for collectability for our mortgage insurer counterparties' inability to fully pay the contractual mortgage insurance claims.

No definition available.

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

Mortgage Insurance Coverage 'risk in force' represents our maximum potential loss recovery under the applicable mortgage insurance policies.

No definition available.

Name:	fnm_MortgageInsuranceCoverageRiskInForce
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents our maximum potential loss recovery under the applicable pool mortgage insurance policies.

No definition available.

Name:	fnm_PoolMortgageInsuranceCoverageRiskInForce
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents our maximum potential loss recovery under the applicable primary mortgage insurance policies.

No definition available.

Name:	fnm_PrimaryMortgageInsuranceCoverageRiskInForce
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount due in settlement of a claim for reimbursement from an insurance company (insurers) and sellers/servicers when the Company has suffered a loss covered under the terms of an insurance policy or a seller/servicer contract.

No definition available.

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

Represents mortgage insurance coverage risk in force as a percentage of unpaid principal balance.

No definition available.

Name:	fnm_TotalMortgageInsuranceCoverageRiskInForceAsPercentageOfUnpaidPrincipalBalanceInGuarantyBookOfBusiness
Namespace Prefix:	fnm_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
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 954
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478785/954-310-50-2

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

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-21

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-20

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20