Summary of Significant Accounting Policies - Regulatory Capital (Details) - USD ($) $ in Billions	Jun. 30, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Deficit of core capital over statutory minimum capital requirement	$ 111.5	$ 124.3

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