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Concentrations of Credit Risk Estimated Benefit from Mortgage Insurers (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Risks and Uncertainties [Abstract]    
Contractual mortgage insurance benefit $ 4,409fnm_InsuranceLossReservesContractualMortgageInsuranceBenefit $ 6,751fnm_InsuranceLossReservesContractualMortgageInsuranceBenefit
Less: Collectability adjustment 290fnm_InsuranceLossReservesAdjustmentForCollectability 431fnm_InsuranceLossReservesAdjustmentForCollectability
Estimated benefit included in total loss reserves $ 4,119fnm_InsuranceLossReservesContractualMortgageInsuranceBenefitNet $ 6,320fnm_InsuranceLossReservesContractualMortgageInsuranceBenefitNet