Earnings (Loss) Per Share (Details) (USD $)	0 Months Ended	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 28, 2013	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Earnings Per Share [Abstract]
Net income (loss)		$ 10,095,000,000		$ 5,119,000,000		$ 68,780,000,000		$ 7,837,000,000
Less: Net (income) loss attributable to noncontrolling interest		(11,000,000)		(5,000,000)		(11,000,000)		(4,000,000)
Net income (loss) attributable to Fannie Mae		10,084,000,000		5,114,000,000		68,769,000,000		7,833,000,000
Dividends distributed to senior preferred stockholder				(2,929,000,000)	[1]			(5,746,000,000)	[1]
Dividends distributed or available for distribution to senior preferred stockholder		(10,243,000,000)	[1]			(69,611,000,000)	[1]
Net income (loss) attributable to common stockholders		(159,000,000)		2,185,000,000		(842,000,000)		2,087,000,000
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		0		131,000,000		0		131,000,000
Weighted-average common shares outstanding, Diluted		5,762,000,000	[2]	5,893,000,000	[2]	5,762,000,000	[2]	5,893,000,000	[2]
Earnings (loss) per share, Basic		$ (0.03)		$ 0.38		$ (0.15)		$ 0.36
Earnings (loss) per share, Diluted		$ (0.03)		$ 0.37		$ (0.15)		$ 0.35
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	59,400,000,000	59,400,000,000				63,592,000,000		5,750,000,000
Expected undeclared dividends payable on senior preferred stock for the next quarter		$ 10,200,000,000

[1] Represents our required dividend payments to Treasury under the terms of the senior preferred stock purchase agreement. For the three months ended June 30, 2013, the dividend is calculated based on our net worth as of June 30, 2013 less the applicable capital reserve amount of $3.0 billion and for the six months ended June 30, 2013, we add dividends paid related to 2013 to this amount. For the three and six months ended June 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 June 30, 2013.