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Concentrations of Credit Risk Geographic Concentration (Details) (Guaranty Book of Business [Member])
Dec. 31, 2014
Dec. 31, 2013
Multifamily [Member]    
Concentration Risk [Line Items]    
Percentage of loans with detailed loan level information 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
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
Multifamily [Member] | Midwest [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 9.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_MidwestUnitedStatesMember		 9.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_MidwestUnitedStatesMember
Multifamily [Member] | Northeast [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 18.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_NortheastUnitedStatesMember		 20.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_NortheastUnitedStatesMember
Multifamily [Member] | Southeast [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 22.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SoutheastUnitedStatesMember		 21.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SoutheastUnitedStatesMember
Multifamily [Member] | Southwest [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 20.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SouthwestUnitedStatesMember		 19.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SouthwestUnitedStatesMember
Multifamily [Member] | West [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
Conventional Mortgage [Member] | Single-Family [Member]    
Concentration Risk [Line Items]    
Percentage of loans with detailed loan level information 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember		 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
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 Mortgage [Member] | Single-Family [Member] | Midwest [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 15.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_MidwestUnitedStatesMember		 15.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_MidwestUnitedStatesMember
Conventional Mortgage [Member] | Single-Family [Member] | Northeast [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 19.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_NortheastUnitedStatesMember		 19.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_NortheastUnitedStatesMember
Conventional Mortgage [Member] | Single-Family [Member] | Southeast [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 22.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SoutheastUnitedStatesMember		 22.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SoutheastUnitedStatesMember
Conventional Mortgage [Member] | Single-Family [Member] | Southwest [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 16.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SouthwestUnitedStatesMember		 16.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
/ us-gaap_StatementGeographicalAxis
= fnm_SouthwestUnitedStatesMember
Conventional Mortgage [Member] | Single-Family [Member] | West [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
Minimum [Member] | Conventional Mortgage [Member] | Single-Family [Member]    
Concentration Risk [Line Items]    
Percentage of loans with detailed loan level information 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember		 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= fnm_ConventionalMortgageMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember