Employee Retirement Benefits (Narratives) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employee Retirement Benefit Plan Net Periodic Benefit Cost	$ 114	$ 94	$ 133

The total amount of net periodic benefit cost for employee retirement benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

No definition available.

Name:	fnm_EmployeeRetirementBenefitPlanNetPeriodicBenefitCost
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration