Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) contracts	Mar. 31, 2019 USD ($) contracts
Financing Receivable, Troubled Debt Restructuring [Line Items]
Average term extension of a single-family modified loan	168 months	157 months
Average interest rate reduction of a single-family modified loan	0.32%	0.10%
Number of loans troubled debt restructurings activity \| contracts	12,610	13,896
Recorded investment troubled debt restructurings activity \| $	$ 2,099	$ 2,112
Single-family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	12,610	13,893
Recorded investment troubled debt restructurings activity \| $	$ 2,099	$ 2,099
Multifamily [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	0	3
Recorded investment troubled debt restructurings activity \| $	$ 0	$ 13
Fixed Rate Residential Mortgage 30 and 20 year Member \| Single-family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	10,856	11,445
Recorded investment troubled debt restructurings activity \| $	$ 1,909	$ 1,826
FixedRateResidentialMortgage15yearMember \| Single-family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	1,073	1,295
Recorded investment troubled debt restructurings activity \| $	$ 98	$ 114
Other [Member] \| Single-family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	537	936
Recorded investment troubled debt restructurings activity \| $	$ 68	$ 127
Adjustable Rate Residential Mortgage \| Single-family [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans troubled debt restructurings activity \| contracts	144	217
Recorded investment troubled debt restructurings activity \| $	$ 24	$ 32

The following represents the average interest rate reduction (disclosed as a %) resulting from loan modifications.

No definition available.

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- Definition

The following represents the average term extension (disclosed in months) resulting from loan modifications.

+ References

No definition available.

+ Details

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Namespace Prefix:	fnm_
Data Type:	xbrli:durationItemType
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- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

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