Impact of the change in conclusion to consolidate a Variable Interest Entity (VIE) in the most recent financial statements (for example, the VIE was previously consolidated and is not currently consolidated) and the dollar amount of the change on the reporting entity's financial statements.

No definition available.

Name:	fnm_VariableInterestEntityConclusiontoConsolidateChangeinFactsandCircumstancesAmount
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Consolidations and Transfers of Financial Assets Types of VIEs (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Consolidated VIEs [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity, Conclusion to Consolidate, Change in Facts and Circumstances, Amount	$ 40
Unconsolidated VIEs [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity, Conclusion to Consolidate, Change in Facts and Circumstances, Amount		$ 559