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

Mortgage Insurance Coverage Risk in Force, Insurance Claim, Percentage To be Paid in Cash by Mortgage Insurer

No definition available.

Name:	fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Amount represents risk in force mortgage insurance coverage provided by the mortgage insurers with publicly disclosed financial weaknesses.

No definition available.

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

Represents the number of insurance companies who provided the majority of our mortgage insurance.

No definition available.

Name:	fnm_NumberOfInsuranceCompaniesWhoProvidedMajorityOfMortgageInsurance
Namespace Prefix:	fnm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of our largest mortgage servicers, including their affiliates, servicing the majority of our guaranty book of business.

No definition available.

Name:	fnm_NumberOfLargestMortgageServicersServicingMajorityOfGuarantyBookOfBusiness
Namespace Prefix:	fnm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of mortgage insurers that have publicly disclosed credit quality deterioration.

No definition available.

Name:	fnm_NumberOfMortgageInsurersPubliclyDisclosedCreditQualityDeterioration
Namespace Prefix:	fnm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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.

No definition available.

Name:	fnm_PercentageOfConcentrationRisk
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
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:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

Name:	fnm_RiskCharacteristicsOfOurBookOfBusinessAxis=fnm_GuarantyBookOfBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CounterpartyNameAxis=fnm_MortgageSellersAndServicersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: