Mortgage Loans Individually Impaired Loans - 2 (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2019
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	$ 85,408	$ 91,876
Individually impaired loans with no related allowance recorded: total interest income recognized	325	905
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	879	2,868
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	74	255
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	44	117
Individually impaired loans with no related allowance recorded: average recorded investment	19,584	18,965
Impaired Financing Receivable, Average Recorded Investment	104,992	110,841
Individually impaired loans: total interest income recognized	1,204	3,773
Impaired Financing Receivable, Interest Income, Cash Basis Method	118	372
Single-family [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	85,095	91,596
Individually impaired loans with no related allowance recorded: total interest income recognized	317	889
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	877	2,861
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	74	255
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	44	117
Individually impaired loans with no related allowance recorded: average recorded investment	19,183	18,588
Single-family [Member] \| Other [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	16,011	17,995
Individually impaired loans with no related allowance recorded: total interest income recognized	55	158
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	164	557
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	11	41
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	5	14
Individually impaired loans with no related allowance recorded: average recorded investment	2,767	2,875
Single-family [Member] \| FixedRateResidentialMortgage15yearMember
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	1,141	1,237
Individually impaired loans with no related allowance recorded: total interest income recognized	4	11
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	10	32
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	1	3
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	1	3
Individually impaired loans with no related allowance recorded: average recorded investment	329	320
Single-family [Member] \| Adjustable Rate Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	155	135
Individually impaired loans with no related allowance recorded: total interest income recognized	3	12
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	1	4
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	1	1
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	1	2
Individually impaired loans with no related allowance recorded: average recorded investment	300	352
Single-family [Member] \| Fixed Rate Residential Mortgage
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	67,788	72,229
Individually impaired loans with no related allowance recorded: total interest income recognized	255	708
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	702	2,268
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	61	210
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	37	98
Individually impaired loans with no related allowance recorded: average recorded investment	15,787	15,041
Multifamily [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: average recorded investment	313	280
Individually impaired loans with no related allowance recorded: total interest income recognized	8	16
Table Footnote [Abstract]
Individually impaired loans with related allowance recorded: total interest income recognized	2	7
Individually impaired loans with related allowance recorded: interest income recognized on a cash basis	0	0
Individually impaired loans with no related allowance recorded: interest income recognized on a cash basis	0	0
Individually impaired loans with no related allowance recorded: average recorded investment	$ 401	$ 377

Represents a tabular schedule for the footnotes to a table.

No definition available.

Name:	fnm_TableFootnoteAbstract
Namespace Prefix:	fnm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of interest income, recognized on cash basis, on financing receivables, for which there is no related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of interest income on financing receivables, for which there is a related allowance for credit losses, recognized on a cash-basis method of accounting that the financing receivables were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeCashBasisMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_SingleFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnm_OtherMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnm_FixedRateResidentialMortgage15yearMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AdjustableRateResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_FixedRateResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: