Consolidations and Transfers of Financial Assets Types of VIEs (Details) $ in Millions	Dec. 31, 2018 USD ($)
Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Conclusion to Consolidate, Change in Facts and Circumstances, Amount	$ 1,500

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