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Investments in Securities Trading 1 (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value $ 31,286us-gaap_TradingSecuritiesDebt $ 31,504us-gaap_TradingSecuritiesDebt
Mortgage-related securities [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 11,919us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
12,038us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Fannie Mae [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 4,959us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
4,940us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Freddie Mac [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 1,723us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
1,369us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Ginnie Mae [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 214us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
166us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Alt-A private-label securities [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 833us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
920us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Subprime private-label securities [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 876us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
1,307us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
CMBS [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 2,478us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
2,515us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Mortgage revenue bonds [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 742us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
722us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Other mortgage-related securities [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value 94us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
99us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
U.S. Treasury securities [Member]    
Schedule of Investments in Trading Securities [Line Items]    
Trading, at fair value $ 19,367us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
$ 19,466us-gaap_TradingSecuritiesDebt
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember