Name:	fnm_NetTransfersFromAdvancestoLenderstoLoansHeldforInvestmentofConsolidatedTrusts1
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents net transfers to (from) mortgage loans held for investment of consolidated trusts from (to) mortgage loans held for investment of Fannie Mae.

No definition available.

Name:	fnm_TransfersFromLoansHeldForInvestmentOfParentCompanyToLoansHeldForInvestmentOfConsolidatedTrusts
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-4

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-3

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-5

Name:	us-gaap_NoncashOrPartNoncashAcquisitionDebtAssumed1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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.

No definition available.

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

Value transferred from mortgage loans to real estate owned (REO) in noncash transactions.