Earnings (Loss) Per Share (Details) (USD $)	3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Earnings Per Share [Abstract]
Net income (loss)	$ 8,744,000,000		$ 1,813,000,000		$ 77,524,000,000		$ 9,650,000,000
Less: Net (income) loss attributable to noncontrolling interest	(7,000,000)		8,000,000		(18,000,000)		4,000,000
Net income (loss) attributable to Fannie Mae	8,737,000,000		1,821,000,000		77,506,000,000		9,654,000,000
Dividends distributed or available for distribution to senior preferred stockholder	(8,617,000,000)	[1]	(2,929,000,000)	[1]	(78,228,000,000)	[1]	(8,675,000,000)	[1]
Net income (loss) attributable to common stockholders	120,000,000		(1,108,000,000)		(722,000,000)		979,000,000
Weighted-average common shares outstanding, Basic	5,762,000,000	[2]	5,762,000,000	[2]	5,762,000,000	[2]	5,762,000,000	[2]
Incremental Common Shares Attributable to Conversion of Preferred Stock	131,000,000		0		0		131,000,000
Weighted-average common shares outstanding, Diluted	5,893,000,000	[2]	5,762,000,000	[2]	5,762,000,000	[2]	5,893,000,000	[2]
Earnings (loss) per share, Basic	$ 0.02		$ (0.19)		$ (0.13)		$ 0.17
Earnings (loss) per share, Diluted	$ 0.02		$ (0.19)		$ (0.13)		$ 0.17
Capital reserve amount for 2013 per the senior preferred stock purchase agreement amendment	3,000,000,000				3,000,000,000
Senior preferred stock dividend rate in effect through December 31, 2012									10.00%
Weighted-average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury	4,600,000,000		4,600,000,000		4,600,000,000		4,600,000,000
Payments of cash dividends on senior preferred stock to Treasury	10,200,000,000				73,835,000,000		8,679,000,000
Expected undeclared dividends payable on senior preferred stock for the next quarter	$ 8,600,000,000

[1] Represents our required dividend payments to Treasury under the terms of the senior preferred stock purchase agreement. For the three months ended September 30, 2013, the dividend is calculated based on our net worth as of September 30, 2013 less the applicable capital reserve amount of $3.0 billion and for the nine months ended September 30, 2013, we add dividends paid related to 2013 to this amount. For the three and nine months ended September 30, 2012, an annual dividend rate of 10% on the aggregate liquidation preference was used to calculate the dividend.

[2] Includes 4.6 billion of weighted-average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury from the date the warrant was issued through September 30, 2013.