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Investments in Securities Available-for-sale Securities 3 (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses $ (4)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4 $ (223)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
Less Than 12 Consecutive Months, Fair Value 339us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 3,580us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
12 Consecutive Moths or Longer, Gross Unrealized Losses (203)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3 (660)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
12 Consecutive Months or Longer, Fair Value 2,375us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 3,802us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Fannie Mae [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses 0fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
(40)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Less Than 12 Consecutive Months, Fair Value 113us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
975us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
12 Consecutive Moths or Longer, Gross Unrealized Losses (19)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
(4)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
12 Consecutive Months or Longer, Fair Value 627us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
126us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Alt-A private-label securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses (2)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
(12)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Less Than 12 Consecutive Months, Fair Value 171us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
490us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
12 Consecutive Moths or Longer, Gross Unrealized Losses (13)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
(30)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
12 Consecutive Months or Longer, Fair Value 112us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
308us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Subprime private-label securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses 0fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
(24)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Less Than 12 Consecutive Months, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
448us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
12 Consecutive Moths or Longer, Gross Unrealized Losses (24)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
(131)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
12 Consecutive Months or Longer, Fair Value 460us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
1,332us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Mortgage revenue bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses (2)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(147)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Less Than 12 Consecutive Months, Fair Value 47us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
1,662us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12 Consecutive Moths or Longer, Gross Unrealized Losses (11)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(277)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12 Consecutive Months or Longer, Fair Value 155us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
970us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Other mortgage-related securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Consecutive Months, Gross Unrealized Losses 0fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
0fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessthan12MonthsAggregateLosses4
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
Less Than 12 Consecutive Months, Fair Value 8us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
5us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
12 Consecutive Moths or Longer, Gross Unrealized Losses (136)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
(218)fnm_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLosses3
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
12 Consecutive Months or Longer, Fair Value $ 1,021us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember
$ 1,066us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember