Summary of Significant Accounting Policies Advances to Lenders (Details)	12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]
Maximum term of early lender funding advances	60 days

Represent the maximum number of days the lenders have when we provide early funding to lenders for loans that they will subsequently either sell to us or securitize into a Fannie Mae MBS that they will deliver to us.

No definition available.

Name:	fnm_EarlyLenderFundingAdvancesMaximumTerm
Namespace Prefix:	fnm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration