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Mortgage Loans Credit Quality Indicators - MF (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans $ 3,036,567fnm_RecordedInvestmentMortgageLoansHeldForInvestment $ 3,047,794fnm_RecordedInvestmentMortgageLoansHeldForInvestment
Multifamily [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 193,028fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
189,233fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Multifamily [Member] | Pass [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 186,052fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
182,079fnm_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 in loans 3,015fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
3,070fnm_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 in loans 3,953fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
3,842fnm_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 in loans $ 8fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
$ 242fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DoubtfulMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember