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Fair Value Measurement (Narratives) (Details) (HARP Loans [Member], USD $)
In Billions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]    
Loans in portfolio that have been refinanced under HARP $ 314.0fnm_LoansReceivableRefinancedUnderHomeAffordableRefinanceProgramFairValueDisclosure $ 306.9fnm_LoansReceivableRefinancedUnderHomeAffordableRefinanceProgramFairValueDisclosure
Fair Value Estimation Process [Member]
    
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]    
Changes in estimated fair value $ 3.3fnm_ChangeInEstimatedFairValueResultingFromChangeInPrincipalMarkets
/ us-gaap_ChangeInAccountingEstimateByTypeAxis
= fnm_FairValueEstimationProcessMember
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember		 $ 11.5fnm_ChangeInEstimatedFairValueResultingFromChangeInPrincipalMarkets
/ us-gaap_ChangeInAccountingEstimateByTypeAxis
= fnm_FairValueEstimationProcessMember
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember