Summary of Significant Accounting Policies Reserve for Guaranty Losses (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Reserve for Guaranty Losses	$ 370	$ 639

The reserve for guaranty losses is a liability account that reflects an estimate of incurred credit losses related to the guaranty of each unconsolidated Fannie Mae MBS trust and other credit guarantees.

No definition available.

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