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Mortgage Loans Credit Quality Indicators - MF (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment $ 3,047,794fnm_RecordedInvestmentMortgageLoansHeldForInvestment $ 3,064,669fnm_RecordedInvestmentMortgageLoansHeldForInvestment
Multifamily [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment 189,233fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 185,978fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Multifamily [Member] | Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment 182,079fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 176,528fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Multifamily [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment 3,070fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 2,234fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Multifamily [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment 3,842fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 6,758fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Multifamily [Member] | Doubtful [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment $ 242fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember		 $ 458fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember