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Concentrations of Credit Risk Risk Characteristics - MF (Details) (Multifamily [Member])
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Guaranty Book of Business [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Percentage of book of business 100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
100.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Percentage of loans with detailed loan level information 99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
99.00%fnm_PercentageOfLoansWithDetailedLoanLevelInformation
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Guaranty Book of Business [Member] | Current Debt Service Coverage Ratio Less Than 1.0 [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Percentage of book of business 3.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_DebtServiceCoverageRatioByCategoryAxis
= fnm_CurrentDebtServiceCoverageRatioLessThanOneHundredPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
4.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_DebtServiceCoverageRatioByCategoryAxis
= fnm_CurrentDebtServiceCoverageRatioLessThanOneHundredPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Loans seriously delinquent, percentage by unpaid principal balance 0.83%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_DebtServiceCoverageRatioByCategoryAxis
= fnm_CurrentDebtServiceCoverageRatioLessThanOneHundredPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
1.09%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_DebtServiceCoverageRatioByCategoryAxis
= fnm_CurrentDebtServiceCoverageRatioLessThanOneHundredPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Guaranty Book of Business [Member] | Original Loan to Value Ratio Greater than 80% [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Percentage of book of business 3.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioGreaterThanEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
3.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioGreaterThanEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Loans seriously delinquent, percentage by unpaid principal balance 0.31%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioGreaterThanEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
0.23%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioGreaterThanEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Guaranty Book of Business [Member] | Original Loan to Value Ratio Less than or Equal to 80% [Member]
   
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Percentage of book of business 97.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioLessThanOrEqualToEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
97.00%fnm_PercentageOfUnpaidPrincipalBalanceOfLoans
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioLessThanOrEqualToEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Loans seriously delinquent, percentage by unpaid principal balance 0.04%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioLessThanOrEqualToEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
0.10%fnm_LoansSeriouslyDelinquentUnpaidPrincipalBalancePercentageByCategory
/ fnm_LoanToValueRatioAtOriginationAxis
= fnm_OriginalLoanToValueRatioLessThanOrEqualToEightyPercentMember
/ fnm_RiskCharacteristicsOfOurBookOfBusinessAxis
= fnm_GuarantyBookOfBusinessMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_MultifamilyMember
Minimum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Serious Delinquency Days past due 60 days 60 days
Current Debt service coverage ratio reporting lag 3 months 3 months
Maximum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Current Debt service coverage ratio reporting lag 6 months 6 months