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Fair Value Fair Value of Financial Instruments (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Financial assets [Abstract]    
Derivative assets, gross amount offset $ (5,626,000,000)us-gaap_DerivativeAssetFairValueGrossLiability $ (5,186,000,000)us-gaap_DerivativeAssetFairValueGrossLiability
Financial liabilities [Abstract]    
Derivative liabilities, gross amount offset (12,036,000,000)us-gaap_DerivativeLiabilityFairValueGrossAsset (10,194,000,000)us-gaap_DerivativeLiabilityFairValueGrossAsset
Fair Value Measurement [Domain]    
Financial assets [Abstract]    
Derivative assets, gross amount offset (5,626,000,000)us-gaap_DerivativeAssetFairValueGrossLiability
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain
(5,186,000,000)us-gaap_DerivativeAssetFairValueGrossLiability
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain
Financial liabilities [Abstract]    
Derivative liabilities, gross amount offset (12,036,000,000)us-gaap_DerivativeLiabilityFairValueGrossAsset
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain
(10,194,000,000)us-gaap_DerivativeLiabilityFairValueGrossAsset
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_FairValueDisclosureItemAmountsDomain
Carrying Value [Member]    
Financial assets [Abstract]    
Cash and cash equivalents and restricted cash 65,299,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
54,565,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Federal funds sold and securities purchased under agreements to resell or similar arrangements 20,230,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
30,950,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Trading Securities 31,286,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
31,504,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Available-for-sale securities 27,732,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
30,654,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Mortgage loans held for sale 613,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
331,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Mortgage loans held for investment, at fair value 3,012,503,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
3,019,163,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Advances to lenders 6,361,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
5,559,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Derivatives assets at fair value 1,692,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
1,485,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Guaranty assets and buy-ups 201,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
210,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Total financial assets 3,165,917,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
3,174,421,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Financial liabilities [Abstract]    
Federal funds purchased and securities loaned or sold under agreements to repurchase 87,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
50,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Derivative liabilities at fair value 802,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
614,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Guaranty obligations 367,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
382,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Total financial liabilities 3,213,887,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
3,223,201,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Estimated Fair Value [Member]    
Financial assets [Abstract]    
Cash and cash equivalents and restricted cash 65,299,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
54,565,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Federal funds sold and securities purchased under agreements to resell or similar arrangements 20,230,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
30,950,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Trading Securities 31,286,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
31,504,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Available-for-sale securities 27,732,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
30,654,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Mortgage loans held for sale 720,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
338,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Mortgage loans held for investment, at fair value 3,094,784,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
3,088,086,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Advances to lenders 6,344,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
5,549,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Derivatives assets at fair value 1,692,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
1,485,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Guaranty assets and buy-ups 599,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
616,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Total financial assets 3,248,686,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
3,243,747,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Financial liabilities [Abstract]    
Federal funds purchased and securities loaned or sold under agreements to repurchase 87,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
50,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Derivative liabilities at fair value 802,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
614,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Guaranty obligations 1,536,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
1,579,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Total financial liabilities 3,321,059,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
3,312,687,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Financial assets [Abstract]    
Cash and cash equivalents and restricted cash 46,349,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
37,965,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Federal funds sold and securities purchased under agreements to resell or similar arrangements 0us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Trading Securities 19,367,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
19,466,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Available-for-sale securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Mortgage loans held for sale 0us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Mortgage loans held for investment, at fair value 0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Advances to lenders 0fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Derivatives assets at fair value 0us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Guaranty assets and buy-ups 0fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Total financial assets 65,716,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
57,431,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Financial liabilities [Abstract]    
Federal funds purchased and securities loaned or sold under agreements to repurchase 0us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Derivative liabilities at fair value 0us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Guaranty obligations 0us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Total financial liabilities 0us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Significant Other Observable Inputs (Level 2) [Member]    
Financial assets [Abstract]    
Cash and cash equivalents and restricted cash 18,950,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
16,600,000,000fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Federal funds sold and securities purchased under agreements to resell or similar arrangements 20,230,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
30,950,000,000us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Trading Securities 9,633,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
9,008,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Available-for-sale securities 14,261,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
15,574,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Mortgage loans held for sale 153,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
169,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Mortgage loans held for investment, at fair value 2,675,585,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
2,687,759,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Advances to lenders 5,901,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
5,079,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Derivatives assets at fair value 7,103,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
6,489,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Guaranty assets and buy-ups 0fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
0fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Total financial assets 2,751,816,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
2,771,628,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Financial liabilities [Abstract]    
Federal funds purchased and securities loaned or sold under agreements to repurchase 87,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
50,000,000us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Derivative liabilities at fair value 12,689,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
10,671,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Guaranty obligations 0us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
0us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Total financial liabilities 3,306,127,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
3,299,289,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Significant Unobservable Inputs (Level 3) [Member]    
Financial assets [Abstract]    
Cash and cash equivalents and restricted cash 0fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0fnm_CashCashEquivalentsAndRestrictedCashFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Federal funds sold and securities purchased under agreements to resell or similar arrangements 0us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_FederalFundsSoldAndSecuritiesBorrowedOrPurchasedUnderAgreementsToResellFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Trading Securities 2,286,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
3,030,000,000us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Available-for-sale securities 13,471,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
15,080,000,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Mortgage loans held for sale 567,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
169,000,000us-gaap_MortgagesHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Mortgage loans held for investment, at fair value 419,199,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
400,327,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Advances to lenders 443,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
470,000,000fnm_AdvancesToLendersFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Derivatives assets at fair value 215,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
182,000,000us-gaap_DerivativeAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Guaranty assets and buy-ups 599,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
616,000,000fnm_GuarantyAssetsAndBuyUpsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Total financial assets 436,780,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
419,874,000,000us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Financial liabilities [Abstract]    
Federal funds purchased and securities loaned or sold under agreements to repurchase 0us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_FederalFundsPurchasedAndSecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Derivative liabilities at fair value 149,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
137,000,000us-gaap_DerivativeLiabilities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Guaranty obligations 1,536,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
1,579,000,000us-gaap_GuaranteesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Total financial liabilities 26,968,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
23,592,000,000us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Fannie Mae [Member]    
Financial liabilities [Abstract]    
Long-term debt 7,920,000,000us-gaap_LongTermDebtFairValue
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
6,403,000,000us-gaap_LongTermDebtFairValue
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Carrying Value [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 235,827,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
239,243,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Financial liabilities [Abstract]    
Short-term debt 99,349,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
105,012,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Long-term debt 349,391,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
355,431,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Estimated Fair Value [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 245,620,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
246,960,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Financial liabilities [Abstract]    
Short-term debt 99,361,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
105,022,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Long-term debt 364,352,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
368,685,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Financial liabilities [Abstract]    
Short-term debt 0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Long-term debt 0us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
0us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 31,127,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
29,896,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Financial liabilities [Abstract]    
Short-term debt 99,361,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
105,022,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Long-term debt 363,358,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
367,703,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 214,493,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
217,064,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Financial liabilities [Abstract]    
Short-term debt 0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Long-term debt 994,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
982,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Consolidated Trusts [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 15,570,000,000us-gaap_LoansReceivableFairValueDisclosure
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
15,629,000,000us-gaap_LoansReceivableFairValueDisclosure
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Long-term debt 21,373,000,000us-gaap_LongTermDebtFairValue
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
19,483,000,000us-gaap_LongTermDebtFairValue
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Consolidated Trusts [Member] | Carrying Value [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 2,776,676,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,779,920,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Short-term debt 1,463,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
1,560,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Long-term debt 2,762,428,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,760,152,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Consolidated Trusts [Member] | Estimated Fair Value [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 2,849,164,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,841,126,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Short-term debt 1,463,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
1,560,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Long-term debt 2,853,458,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,835,177,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Consolidated Trusts [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Short-term debt 0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Long-term debt 0us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Consolidated Trusts [Member] | Significant Other Observable Inputs (Level 2) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 2,644,458,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,657,863,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Short-term debt 0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Long-term debt 2,830,632,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,815,843,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member]    
Financial assets [Abstract]    
Mortgage loans held for investment, at fair value 204,706,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
183,263,000,000us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Financial liabilities [Abstract]    
Short-term debt 1,463,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
1,560,000,000us-gaap_ShorttermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Long-term debt 22,826,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
19,334,000,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
HARP loans [Member]    
Financial liabilities [Abstract]    
Loans Receivable Refinanced under Home Affordable Refinance Program, Fair Value Disclosure 310,200,000,000fnm_LoansReceivableRefinancedUnderHomeAffordableRefinanceProgramFairValueDisclosure
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember
314,000,000,000fnm_LoansReceivableRefinancedUnderHomeAffordableRefinanceProgramFairValueDisclosure
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember
HARP loans [Member] | Fair Value Estimation Process [Member]    
Financial liabilities [Abstract]    
Change In Estimated Fair Value Resulting From Change In Principal Market $ 3,500,000,000fnm_ChangeInEstimatedFairValueResultingFromChangeInPrincipalMarket
/ us-gaap_ChangeInAccountingEstimateByTypeAxis
= fnm_FairValueEstimationProcessMember
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember
$ 3,300,000,000fnm_ChangeInEstimatedFairValueResultingFromChangeInPrincipalMarket
/ us-gaap_ChangeInAccountingEstimateByTypeAxis
= fnm_FairValueEstimationProcessMember
/ fnm_NewAccountPronouncementEffectOfChangeOnAssetsByTypeAxis
= fnm_HomeAffordableRefinanceProgramHarpLoansMember