Commitments and Contingencies (Details) (USD $) In Billions, unless otherwise specified	12 Months Ended	0 Months Ended				1 Months Ended						0 Months Ended	1 Months Ended	0 Months Ended
	Dec. 31, 2008 Pending Litigation [Member] 2008 Class Action Lawsuits [Member] cases	Oct. 22, 2012 Pending Litigation [Member] In re 2008 Fannie Mae ERISA Litigation [Member] defendants	Mar. 02, 2012 Pending Litigation [Member] In re 2008 Fannie Mae ERISA Litigation [Member] defendants	Aug. 30, 2012 Pending Litigation [Member] Comprehensive Investment Services v. Mudd, et al. [Member] defendants	Jul. 30, 2008 Pending Litigation [Member] Housing Trust Fund [Member]	Dec. 31, 2010 Unasserted Claim [Member] Minimum [Member] Pending Litigation [Member] In re Fannie Mae Securities Litigation [Member]	Sep. 30, 2010 Unasserted Claim [Member] Minimum [Member] Pending Litigation [Member] In re Fannie Mae Securities Litigation [Member]	Dec. 31, 2010 Unasserted Claim [Member] Maximum [Member] Pending Litigation [Member] In re Fannie Mae Securities Litigation [Member]	Sep. 30, 2010 Unasserted Claim [Member] Maximum [Member] Pending Litigation [Member] In re Fannie Mae Securities Litigation [Member]	May 31, 2008 Series T Preferred Stock [Member] Pending Litigation [Member] Comprehensive Investment Services v. Mudd, et al. [Member]	Dec. 31, 2007 Series S Preferred Stock [Member] Pending Litigation [Member] Smith v. Fannie Mae, et al. [Member]	May 20, 2013 Michigan Pending Litigation [Member] Transfer Tax Litigation [Member] cases	Mar. 31, 2012 Michigan Pending Litigation [Member] Transfer Tax Litigation [Member] cases	Jul. 30, 2013 District of Columbia Senior Preferred Stock Purchase Agreements Litigation [Member] Subsequent Event [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													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 Which the Company Received Exemption from State Transfer Taxes												2	2
Basis points of each dollar of unpaid principal balance					0.042%
Percentage of the funds set aside as required by the Federal Housing Financie Regulatory Reform Act of 2008 that should be allocated to the Housing Trust Fund					65.00%
Percentage of the funds set aside as required by the Federal Housing Finance Regulatory Reform Act of 2008 that should be allocated to the Capital Magnet Fund					35.00%