Summary of Significant Accounting Policies Regulatory Capital (Details) - USD ($) $ in Billions	Jun. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Deficit of core capital over statutory minimum capital requirement	$ 137.9	$ 136.2

Represents the deficit of core capital over statutory minimum capital requirement.

No definition available.

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