Consolidations and Transfers of Financial Assets - Types of VIEs (Details) $ in Millions	Dec. 31, 2021 USD ($)
Not Primary Beneficiary
Variable Interest Entity [Line Items]
VIE, consolidated unpaid principal balance	$ 85

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