Commitments and Contingencies (Details) (Pending Litigation [Member], USD $) In Billions, unless otherwise specified	12 Months Ended	0 Months Ended			1 Months Ended				0 Months Ended		1 Months Ended
	Dec. 31, 2008 2008 Class Action Lawsuits [Member] cases	Oct. 22, 2012 In re 2008 Fannie Mae ERISA Litigation [Member] defendants	Mar. 02, 2012 In re 2008 Fannie Mae ERISA Litigation [Member] defendants	Aug. 30, 2012 Comprehensive Investment Services v. Mudd, et al. [Member] defendants	Dec. 31, 2010 Unasserted Claim [Member] Minimum [Member] In re Fannie Mae Securities Litigation [Member]	Sep. 30, 2010 Unasserted Claim [Member] Minimum [Member] In re Fannie Mae Securities Litigation [Member]	Dec. 31, 2010 Unasserted Claim [Member] Maximum [Member] In re Fannie Mae Securities Litigation [Member]	Sep. 30, 2010 Unasserted Claim [Member] Maximum [Member] In re Fannie Mae Securities Litigation [Member]	May 19, 2008 Series T Preferred Stock [Member] Comprehensive Investment Services v. Mudd, et al. [Member]	Dec. 11, 2007 Series S Preferred Stock [Member] Smith v. Fannie Mae, et al. [Member]	Mar. 31, 2012 MICHIGAN Transfer Tax Litigation [Member] cases
Loss Contingencies [Line Items]
Damages claimed to plaintiffs under various scenarios					$ 2.2	$ 2.2	$ 8.6	$ 8.6
Number of lawsuits that Fannie Mae is a defendant	2
Number of current Board members added as defendants			2
Number of former and current directors and officers dismissed from the action		7		2
Preferred stock offering amount									$ 2.0	$ 7.0
Preferred stock, annual dividend rate									8.25%	7.75%
Number of cases that we are not exempt from Michigan transfer taxes under our charter											2