Summary of Significant Accounting Policies Investments in Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Fair value of guaranty obligation	$ 276	$ 446

The fair value of our guaranty obligations associated with the Fannie Mae MBS included in “Investments in securities”

No definition available.

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