Fair Value Fair Value Measurement (Narratives) (Details) (HARP loans [Member], USD $) In Billions, unless otherwise specified	Mar. 31, 2013
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Loans in portfolio that have been refinanced under HARP	$ 279.8
Fair Value Estimation Process [Member]
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Changes in estimated fair value	$ (9.2)

The net increase or decrease in fair value as a result of changes in the principal market used to determine fair value.

No definition available.

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