Commitments and Contingencies (Details) (USD $)	12 Months Ended				9 Months Ended	0 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2008 cases	Feb. 25, 2010 cases	Oct. 22, 2012 defendants	Mar. 02, 2012 defendants	Aug. 30, 2012 defendants	May 31, 2008	Dec. 31, 2007	May 19, 2008	Dec. 11, 2007
Loss Contingencies [Line Items]
Rent expenses for operating leases	$ 43,000,000	$ 41,000,000	$ 41,000,000
Series T Preferred Stock [Member]
Loss Contingencies [Line Items]
Preferred stock, annual dividend rate (as a percent)	8.25%
Series S Preferred Stock [Member]
Loss Contingencies [Line Items]
Preferred stock, annual dividend rate (as a percent)	7.75%
Pending Litigation [Member] | Class Action Lawsuits in Year Two Thousand and Eight [Member]
Loss Contingencies [Line Items]
Number of lawsuits that Fannie Mae is a defendant				2
Pending Litigation [Member] | Individual Securities Actions [Member]
Loss Contingencies [Line Items]
Number of lawsuits that Fannie Mae is a defendant					2
Pending Litigation [Member] | In re 2008 Fannie Mae ERISA Litigation [Member]
Loss Contingencies [Line Items]
Number of current Board members added as defendants							2
Number of former and current directors and officers dismissed from the action						7
Pending Litigation [Member] | Comprehensive Investment Services v. Mudd, et al. [Member]
Loss Contingencies [Line Items]
Number of former and current directors and officers dismissed from the action								2
Pending Litigation [Member] | Series T Preferred Stock [Member] | Comprehensive Investment Services v. Mudd, et al. [Member]
Loss Contingencies [Line Items]
Preferred stock offering amount											2,000,000,000
Preferred stock, annual dividend rate (as a percent)									8.25%
Pending Litigation [Member] | Series S Preferred Stock [Member] | Smith v. Fannie Mae, et al. [Member]
Loss Contingencies [Line Items]
Preferred stock offering amount												$ 7,000,000,000
Preferred stock, annual dividend rate (as a percent)										7.75%