Summary of Significant Accounting Policies Employee Retirement Benefits (Details) $ in Millions	1 Months Ended
Jul. 31, 2015 USD ($)
Pension Plan [Member] \| Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement of defined pension benefit obligation, cash contribution	$ 102

The amount of additional cash contributed to settle our defined pension benefit obligations upon transferring asset to annuity provider and distributing lump sum payment to participants based on elections.

No definition available.

Name:	fnm_Settlementofdefinedpensionbenefitobligationcashcontribution
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration