Income Taxes - Benefit for Income Taxes (Details) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Current income tax benefit					$ 0		$ 90,000,000		$ 82,000,000
Deferred income tax benefit					0	[1]	0	[1]	0	[1]
Benefit for federal income taxes	(1,000,000)	0	93,000,000	(2,000,000)	0		90,000,000		82,000,000
Proceeds from income tax refunds							1,100,000,000
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Statutory corporate tax rate					35.00%		35.00%		35.00%
Tax-exempt interest					(0.70%)		0.90%		1.30%
Equity investments in affordable housing projects					(3.90%)		4.80%		6.30%
Other					0.20%		1.00%		0.10%
Valuation allowance					(30.60%)		(41.20%)		(42.10%)
Effective tax rate					0.00%		0.50%		0.60%
Change in income tax provision due to valuation allowance changes					$ (5,300,000,000)		$ 7,000,000,000		$ 5,900,000,000

[1] Amount excludes the income tax effect of items recognized directly in “Fannie Mae stockholders’ equity (deficit).”