Consolidations and Transfers of Financial Assets Types of VIE (Details) (USD $)	Jun. 30, 2012 Consolidated VIEs [Member]	Dec. 31, 2011 Unconsolidated VIEs [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance, consolidation of previously unconsolidated VIE	$ 2,800,000,000
Unpaid principal balance, deconsolidation of previously consolidated VIE		$ 102,000,000

Represents the unpaid principal balance of VIEs which were unconsolidated as of the beginning of the period and consolidated as of the end of the period disclosed, generally due to decreases in the amount of the certificates issued by the entity that are held in our portfolio. As a result of deconsolidating these entities, we derecognized the assets and liabilities of the entities and recognized at fair value our retained interests as securities in our consolidated balance sheet.

No definition available.

Name:	fnm_UnpaidPrincipalBalancePreviouslyConsolidatedVariableInterestEntities
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant