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Mortgage Loans Credit Quality Indicators - SF (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans $ 3,036,567fnm_RecordedInvestmentMortgageLoansHeldForInvestment $ 3,047,794fnm_RecordedInvestmentMortgageLoansHeldForInvestment
Government [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 44,600fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_GovernmentMortgageLoanMember
45,300fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_GovernmentMortgageLoanMember
Single-Family [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,843,539fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
2,858,561fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,648,371fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
2,656,220fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Less Than or Equal to 80% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,126,305fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
2,156,165fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 80% and Less Than or Equal to 90% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 270,619fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
261,709fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 90% and Less Than or Equal to 100% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 154,066fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
140,778fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 100% and Less Than or Equal to 110% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 43,305fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
43,014fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 110% and Less Than or Equal to 120% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 23,264fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
23,439fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 120% and Less Than or Equal to 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 7,445fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
7,529fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Primary [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 23,367fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
23,586fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_PrimaryMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 108,294fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
112,761fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Less Than or Equal to 80% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 58,557fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
60,851fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 80% and Less Than or Equal to 90% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 14,661fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
15,151fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 90% and Less Than or Equal to 100% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 12,135fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
12,490fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 100% and Less Than or Equal to 110% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 8,503fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
8,998fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 110% and Less Than or Equal to 120% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 5,707fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
6,033fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 120% and Less Than or Equal to 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,000fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
2,114fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Alt-A [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 6,731fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
7,124fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_AltAMortgageLoansMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 42,293fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
44,259fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Less Than or Equal to 80% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 21,584fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
22,558fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioLessThanOrEqualTo80PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 80% and Less Than or Equal to 90% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 5,756fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
6,046fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan80PercentAndLessThanOrEqualTo90PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 90% and Less Than or Equal to 100% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 5,023fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
5,236fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan90PercentAndLessThanOrEqualTo100PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 100% and Less Than or Equal to 110% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 3,692fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
3,900fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan100PercentAndLessThanOrEqualTo110PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 110% and Less Than or Equal to 120% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,517fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
2,615fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan110PercentAndLessThanOrEqualTo120PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 120% and Less Than or Equal to 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 848fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
904fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan120PercentAndLessThanOrEqualTo125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Other [Member] | Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 125% [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans 2,873fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
3,000fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_OtherMortgageLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThan125PercentMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
Single-Family [Member] | Government [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total recorded investment in loans $ 44,581fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_GovernmentMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember
$ 45,321fnm_RecordedInvestmentMortgageLoansHeldForInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fnm_GovernmentMortgageLoanMember
/ us-gaap_StatementBusinessSegmentsAxis
= fnm_SingleFamilyMember