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Short-Term Borrowings and Long-Term Debt Short-term Debt (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Short-term Borrowings [Line Items]    
Federal funds purchased and securities sold under agreements to repurchase 50,000,000us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchase 0us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchase
Federal funds purchased and securities sold under agreements to repurchase, Weighted-average Interest Rate 0.00%fnm_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchaseWeightedAverageInterestRate 0.00%fnm_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchaseWeightedAverageInterestRate
Short-term debt, outstanding 106,572,000,000us-gaap_ShortTermBorrowings 74,449,000,000us-gaap_ShortTermBorrowings
Short-term debt, Weighted-average Interest Rate 0.11%us-gaap_ShortTermDebtWeightedAverageInterestRate 0.13%us-gaap_ShortTermDebtWeightedAverageInterestRate
Secured Uncommitted Line of Credit [Member]    
Short-term Borrowings [Line Items]    
Line of credit maximum borrowing capacity 15,000,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= fnm_SecuredUncommittedLineOfCreditMember
20,000,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= fnm_SecuredUncommittedLineOfCreditMember
Line of credit amount outstanding 0us-gaap_LineOfCredit
/ us-gaap_ShortTermDebtTypeAxis
= fnm_SecuredUncommittedLineOfCreditMember
 
Fannie Mae [Member]    
Short-term Borrowings [Line Items]    
Short-term debt, outstanding 105,012,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
72,295,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Short-term debt, Weighted-average Interest Rate 0.11%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
0.13%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]    
Short-term Borrowings [Line Items]    
Short-term debt, outstanding 105,012,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateDiscountNoteMember
71,933,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateDiscountNoteMember
Short-term debt, Weighted-average Interest Rate 0.11%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateDiscountNoteMember
0.12%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateDiscountNoteMember
Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]    
Short-term Borrowings [Line Items]    
Short-term debt, outstanding 0us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateForeignExchangeDiscountNoteMember
362,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateForeignExchangeDiscountNoteMember
Short-term debt, Weighted-average Interest Rate 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateForeignExchangeDiscountNoteMember
1.07%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= fnm_FixedRateForeignExchangeDiscountNoteMember
Consolidated Trusts [Member]    
Short-term Borrowings [Line Items]    
Short-term debt, outstanding 1,560,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,154,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Short-term debt, Weighted-average Interest Rate 0.09%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
0.09%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Minimum [Member] | Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]    
Short-term Borrowings [Line Items]    
Short-term debt instrument maturity 5 days 5 days
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]    
Short-term Borrowings [Line Items]    
Short-term debt instrument maturity 360 days 360 days
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]    
Short-term Borrowings [Line Items]    
Short-term debt instrument maturity 360 days 360 days