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Concentrations of Credit Risk (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Jul. 21, 2014
Jul. 20, 2014
Concentration Risk [Line Items]        
Receivable from claims on insured, defaulted loans excluding government insured loans $ 1,400,000,000fnm_ReceivableFromClaimsOnDefaultedLoansFromMortgageInsurersSellersAndServicers $ 2,100,000,000fnm_ReceivableFromClaimsOnDefaultedLoansFromMortgageInsurersSellersAndServicers    
Allowance for mortgage insurance receivable 799,000,000fnm_AllowanceForMortgageInsuranceReceivable 655,000,000fnm_AllowanceForMortgageInsuranceReceivable    
Unpaid principal balance of guaranteed non-agency securities in our portfolio 4,551,000,000fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio 5,554,000,000fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio    
Maximum potential exposure under guarantees not recognized in our consolidated balance sheets 6,500,000,000fnm_GuarantorObligationMaximumExposureOffBalanceSheet 7,300,000,000fnm_GuarantorObligationMaximumExposureOffBalanceSheet    
Maximum amount recoverable for off-balance sheet guarantees through available credit enhancements and recourse with third parties 2,700,000,000fnm_GuaranteeExposureAmountRecoverableFromCreditEnhancementsAndRecourseWithThirdPartiesOffBalanceSheet 3,100,000,000fnm_GuaranteeExposureAmountRecoverableFromCreditEnhancementsAndRecourseWithThirdPartiesOffBalanceSheet    
Fannie Mae [Member] | Senior Floating Connecticut Avenue Security [Member]        
Concentration Risk [Line Items]        
Long-term Debt Issued 5,800,000,000fnm_LongtermDebtIssued
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= fnm_SeniorFloatingConnecticutAvenueSecurityMember
     
Single-Family [Member]        
Concentration Risk [Line Items]        
Maximum potential loss recovery from lenders under risk sharing agreements 8,900,000,000fnm_LossContingencyRelatedReceivableMaximum
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
10,700,000,000fnm_LossContingencyRelatedReceivableMaximum
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Number of mortgage servicers, including their affiliates, which serviced the majority of our guaranty book of business 5fnm_NumberOfLargestMortgageServicersServicingMajorityOfGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
5fnm_NumberOfLargestMortgageServicersServicingMajorityOfGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Mortgage Insurance Coverage Risk In Force 109,600,000,000fnm_MortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
102,500,000,000fnm_MortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Mortgage insurance coverage risk in force as percentage 4.00%fnm_TotalMortgageInsuranceCoverageRiskInForceAsPercentageOfUnpaidPrincipalBalanceInGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
4.00%fnm_TotalMortgageInsuranceCoverageRiskInForceAsPercentageOfUnpaidPrincipalBalanceInGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Primary mortgage insurance coverage risk in force 108,700,000,000fnm_PrimaryMortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
101,400,000,000fnm_PrimaryMortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Pool mortgage insurance coverage risk in force 852,000,000fnm_PoolMortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
1,100,000,000fnm_PoolMortgageInsuranceCoverageRiskInForce
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Number of mortgage insurance companies provided majority of our mortgage insurance 4fnm_NumberOfInsuranceCompaniesWhoProvidedMajorityOfMortgageInsurance
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
4fnm_NumberOfInsuranceCompaniesWhoProvidedMajorityOfMortgageInsurance
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Multifamily [Member]        
Concentration Risk [Line Items]        
Maximum potential loss recovery from lenders under risk sharing agreements 41,700,000,000fnm_LossContingencyRelatedReceivableMaximum
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
39,400,000,000fnm_LossContingencyRelatedReceivableMaximum
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
Number of mortgage servicers, including their affiliates, which serviced the majority of our guaranty book of business 10fnm_NumberOfLargestMortgageServicersServicingMajorityOfGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
10fnm_NumberOfLargestMortgageServicersServicingMajorityOfGuarantyBookOfBusiness
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
Lender Concentration Risk [Member] | Single-Family [Member]        
Concentration Risk [Line Items]        
Number of major lenders with risk sharing agreements 3fnm_ConcentrationRiskNumberOfLendersMaximumPotentialLossRecovery
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
3fnm_ConcentrationRiskNumberOfLendersMaximumPotentialLossRecovery
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Lender Concentration Risk [Member] | Multifamily [Member]        
Concentration Risk [Line Items]        
Number of major lenders with risk sharing agreements 3fnm_ConcentrationRiskNumberOfLendersMaximumPotentialLossRecovery
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
3fnm_ConcentrationRiskNumberOfLendersMaximumPotentialLossRecovery
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
Insurance Service Provider Concentration Risk [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Number of mortgage insurers with publicly disclosed credit quality deterioration 3fnm_NumberOfMortgageInsurersPubliclyDisclosedCreditQualityDeterioration
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
     
Mortgage insurance coverage risk in force, insurers with credit quality deterioration 12,300,000,000fnm_MortgageInsuranceCoverageRiskInForceInsurersWithPubliclyDisclosedFinancialWeakness
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
     
Mortgage Insurance Coverage [Member] | Insurance Service Provider Concentration Risk [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 11.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_MortgageInsuranceCoverageMember
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
     
Group of Top Mortgage Insurance Companies [Member] | Mortgage Insurance Coverage [Member] | Insurance Service Provider Concentration Risk [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 79.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_MortgageInsuranceCoverageMember
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfTopMortgageInsuranceCompaniesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
78.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_MortgageInsuranceCoverageMember
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfTopMortgageInsuranceCompaniesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Group of Largest Mortgage Servicers including Affiliates [Member] | Unpaid Principal Balance [Member] | Credit Concentration Risk [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 46.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfLargestMortgageServicersIncludingAffiliatesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
49.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfLargestMortgageServicersIncludingAffiliatesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Group of Largest Mortgage Servicers including Affiliates [Member] | Unpaid Principal Balance [Member] | Credit Concentration Risk [Member] | Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 67.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfLargestMortgageServicersIncludingAffiliatesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
65.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_GroupOfLargestMortgageServicersIncludingAffiliatesMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
Republic Mortgage Insurance Company and Affiliate [Member]        
Concentration Risk [Line Items]        
Percentage of mortgage guaranty insurance policy claims to be paid by insurer 100.00%fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_RepublicMortgageInsuranceCompanyAndAffiliateMember
     
PMI [Member]        
Concentration Risk [Line Items]        
Percentage of mortgage guaranty insurance policy claims to be paid by insurer 67.00%fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_PmiMortgageInsuranceCoMember
     
Mortgage Insurance Coverage Risk in Force, Insurance Claim, Percentage To be Deferred by Mortgage Insurer 33.00%fnm_MortgageInsuranceCoverageRiskinForceInsuranceClaimPercentageTobeDeferredbyMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_PmiMortgageInsuranceCoMember
     
Triad Guaranty Insurance Corporation [Member]        
Concentration Risk [Line Items]        
Percentage of mortgage guaranty insurance policy claims to be paid by insurer 75.00%fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_TriadGuarantyInsuranceCorporationMember
     
Mortgage Insurance Coverage Risk in Force, Insurance Claim, Percentage To be Deferred by Mortgage Insurer 25.00%fnm_MortgageInsuranceCoverageRiskinForceInsuranceClaimPercentageTobeDeferredbyMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_TriadGuarantyInsuranceCorporationMember
     
Mortgage Sellers and Servicers [Member]        
Concentration Risk [Line Items]        
Receivable from claims on insured, defaulted loans excluding government insured loans 269,000,000fnm_ReceivableFromClaimsOnDefaultedLoansFromMortgageInsurersSellersAndServicers
/ us-gaap_CounterpartyNameAxis
= fnm_MortgageSellersAndServicersMember
402,000,000fnm_ReceivableFromClaimsOnDefaultedLoansFromMortgageInsurersSellersAndServicers
/ us-gaap_CounterpartyNameAxis
= fnm_MortgageSellersAndServicersMember
   
Ambac Assurance Corporation [Member]        
Concentration Risk [Line Items]        
Percentage of mortgage guaranty insurance policy claims to be paid by insurer     45.00%fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_AmbacAssuranceCorporationMember
25.00%fnm_MortgageInsuranceCoverageRiskInForceInsuranceClaimPercentageToBePaidInCashByMortgageInsurer
/ us-gaap_CounterpartyNameAxis
= fnm_AmbacAssuranceCorporationMember
Ambac Assurance Corporation [Member] | Insurance Service Provider Concentration Risk [Member]        
Concentration Risk [Line Items]        
Unpaid principal balance of guaranteed non-agency securities in our portfolio 2,100,000,000fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_AmbacAssuranceCorporationMember
     
Ambac Assurance Corporation [Member] | Non-Government Guarantee [Member] | Insurance Service Provider Concentration Risk [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 45.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_NonGovernmentGuaranteeMember
/ us-gaap_ConcentrationRiskByTypeAxis
= fnm_InsuranceServiceProviderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_AmbacAssuranceCorporationMember
     
Three Major Lenders [Member] | Lenders With Risk Sharing Agreements [Member] | Lender Concentration Risk [Member] | Single-Family [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 47.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_RiskSharingMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_ThreeMajorLendersMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
52.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_RiskSharingMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_ThreeMajorLendersMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Three Major Lenders [Member] | Lenders With Risk Sharing Agreements [Member] | Lender Concentration Risk [Member] | Multifamily [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 32.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_RiskSharingMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_ThreeMajorLendersMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
32.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_RiskSharingMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember
/ us-gaap_CounterpartyNameAxis
= fnm_ThreeMajorLendersMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
   
West [Member] | Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 31.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_WestUnitedStatesMember
31.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_WestUnitedStatesMember
   
California | Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 23.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_CA
24.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_CA
   
Texas | Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 11.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_TX
     
New York | Multifamily [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 11.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_NY
12.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_NY
   
Conventional Loan [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Conventional Loan [Member] | West [Member] | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 28.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_WestUnitedStatesMember
28.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_WestUnitedStatesMember
   
Conventional Loan [Member] | California | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 20.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_CA
20.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_CA
   
Conventional Loan [Member] | New York | Single-Family [Member] | Guaranty Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of book of business 5.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_NY
5.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= stpr_NY
   
Agency [Member]        
Concentration Risk [Line Items]        
Guaranteed securities issued by Freddie Mac, the federal government and its agencies 19,200,000,000fnm_PrincipalAmountOutstandingonGuaranteedAgencySecuritiesHeldinportfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
22,500,000,000fnm_PrincipalAmountOutstandingonGuaranteedAgencySecuritiesHeldinportfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
   
Alt-A [Member] | Credit Concentration Risk [Member] | Single-Family [Member] | Mortgage Credit Book of Business [Member]        
Concentration Risk [Line Items]        
Unpaid principal balance of mortgage loans $ 117,600,000,000fnm_MortgageLoansHeldForInvestmentUnpaidPrincipalBalance
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_CreditBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
$ 132,500,000,000fnm_MortgageLoansHeldForInvestmentUnpaidPrincipalBalance
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_CreditBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
   
Alt-A [Member] | Unpaid Principal Balance [Member] | Credit Concentration Risk [Member] | Single-Family [Member] | Mortgage Credit Book of Business [Member]        
Concentration Risk [Line Items]        
Percentage of concentration risk 4.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_CreditBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
5.00%fnm_PercentageOfConcentrationRisk
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= fnm_UnpaidPrincipalBalanceMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_CreditBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember