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Concentrations of Credit Risk Financial Guarantor (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Concentration Risk [Line Items]    
Unpaid principal balance of guaranteed non-agency securities in our portfolio $ 4,551fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio $ 5,554fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
Alt-A private-label securities [Member]    
Concentration Risk [Line Items]    
Unpaid principal balance of guaranteed non-agency securities in our portfolio 325fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember		 511fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesAltAMember
Subprime private-label securities [Member]    
Concentration Risk [Line Items]    
Unpaid principal balance of guaranteed non-agency securities in our portfolio 820fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember		 868fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Mortgage revenue bonds [Member]    
Concentration Risk [Line Items]    
Unpaid principal balance of guaranteed non-agency securities in our portfolio 3,168fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 3,911fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Other mortgage-related securities [Member]    
Concentration Risk [Line Items]    
Unpaid principal balance of guaranteed non-agency securities in our portfolio $ 238fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember		 $ 264fnm_PrincipalAmountOutstandingonGuaranteedNonAgencySecuritiesHeldinPortfolio
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesOtherMember