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Investments in Securities Available-for-sale Securities 2 (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost $ 27,413us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 35,690us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Gross unrealized gains 3,448fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1 3,364fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
Gross unrealized losses - OTTI (55)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment (385)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
Gross unrealized losses, other (152)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther (498)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
Total fair value 30,654us-gaap_AvailableForSaleSecuritiesDebtSecurities 38,171us-gaap_AvailableForSaleSecuritiesDebtSecurities
Fannie Mae [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 5,330us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
6,227us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Gross unrealized gains 328fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
390fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Gross unrealized losses - OTTI 0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Gross unrealized losses, other (19)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
(44)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Total fair value 5,639us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
6,573us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Freddie Mac [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 5,100us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
6,365us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Gross unrealized gains 428fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
477fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Gross unrealized losses - OTTI 0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Gross unrealized losses, other 0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Total fair value 5,528us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
6,842us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Ginnie Mae [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 416us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
512us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Gross unrealized gains 60fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
76fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Gross unrealized losses - OTTI 0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Gross unrealized losses, other 0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Total fair value 476us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
588us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Alt-A private-label securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 4,638us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
6,240us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Gross unrealized gains 1,055fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
1,151fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Gross unrealized losses - OTTI (15)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
(40)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Gross unrealized losses, other 0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
(2)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Total fair value 5,678us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
7,349us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Subprime private-label securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 4,103us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
6,232us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Gross unrealized gains 1,161fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
991fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Gross unrealized losses - OTTI (9)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
(102)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Gross unrealized losses, other (15)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
(53)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Total fair value 5,240us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
7,068us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
CMBS [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 1,341us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
1,526us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized gains 56fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
80fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized losses - OTTI 0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
0fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross unrealized losses, other 0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
0fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Total fair value 1,397us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
1,606us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Mortgage revenue bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 3,859us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,645us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized gains 177fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
35fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized losses - OTTI (8)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(228)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross unrealized losses, other (5)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(196)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Total fair value 4,023us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
5,256us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Other mortgage-related securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Total amortized cost 2,626us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
2,943us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
Gross unrealized gains 183fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
164fnm_AvailableForSaleSecuritiesGrossUnrealizedGains1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
Gross unrealized losses - OTTI (23)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
(15)fnm_AvailableforsaleSecuritiesGrossUnrealizedLossesOtherthantemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
Gross unrealized losses, other (113)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
(203)fnm_AvailableForSaleSecuritiesGrossUnrealizedLossesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
Total fair value $ 2,673us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
$ 2,889us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember