(Loss) Earnings Per Share (Details) (USD $)	0 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Earnings Per Share [Abstract]
Capital reserve amount for the next fiscal year based on the amended Senior Preferred Stock Purchase agreement	$ 2,400,000,000	$ 2,400,000,000								$ 2,400,000,000
Capital Reserve Amount, current Year, Senior Preferred Stock Purchase Agreement, Amendment	3,000,000,000	3,000,000,000								3,000,000,000
Senior Preferred Stock, Dividend Rate, Percentage												10.00%		10.00%
Net income (loss)		6,458,000,000	8,744,000,000	10,095,000,000	58,685,000,000	7,570,000,000	1,813,000,000	5,119,000,000	2,718,000,000	83,982,000,000		17,220,000,000		(16,855,000,000)
Less: Net (income) loss attributable to noncontrolling interest		(1,000,000)	(7,000,000)	(11,000,000)	0	0	8,000,000	(5,000,000)	1,000,000	(19,000,000)		4,000,000		0
Net income (loss) attributable to Fannie Mae		6,457,000,000	8,737,000,000	10,084,000,000	58,685,000,000	7,570,000,000	1,821,000,000	5,114,000,000	2,719,000,000	83,963,000,000		17,224,000,000		(16,855,000,000)
Dividends distributed or available for distribution to senior preferred stockholder		(7,191,000,000)	(8,617,000,000)	(10,243,000,000)	(59,368,000,000)	(7,152,000,000)	(2,929,000,000)	(2,929,000,000)	(2,817,000,000)	(85,419,000,000)	[1]	(15,827,000,000)	[1]	(9,614,000,000)	[1]
Net (loss) income attributable to common stockholders (Note 11)		(734,000,000)	120,000,000	(159,000,000)	(683,000,000)	418,000,000	(1,108,000,000)	2,185,000,000	(98,000,000)	(1,456,000,000)		1,397,000,000		(26,469,000,000)
Weighted-average common shares outstanding—Basic (shares)		5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,762,000,000	5,761,000,000	5,762,000,000	[2]	5,762,000,000	[2]	5,737,000,000	[2]
Convertible preferred stock (shares)										0		131,000,000		0
Weighted-average common shares outstanding—Diluted (shares)		5,762,000,000	5,893,000,000	5,762,000,000	5,762,000,000	5,893,000,000	5,762,000,000	5,893,000,000	5,761,000,000	5,762,000,000	[2]	5,893,000,000	[2]	5,737,000,000	[2]
Earnings (loss) per share:
Basic (per share)		$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ 0.07	$ (0.19)	$ 0.38	$ (0.02)	$ (0.25)		$ 0.24		$ (4.61)
Diluted (per share)		$ (0.13)	$ 0.02	$ (0.03)	$ (0.12)	$ 0.07	$ (0.19)	$ 0.37	$ (0.02)	$ (0.25)		$ 0.24		$ (4.61)
Weighted-average shares of common stock that would be issued upon the full exercise of the warrant issued to Treasury										4,600,000,000		4,700,000,000		4,600,000,000
Payment Of Cash Dividends On Senior Preferred Stock To Treasury	8,600,000,000	8,600,000,000								82,452,000,000		11,608,000,000		9,613,000,000
Undeclared Dividends On Senior Preferred Stock Next Quarter		$ 7,200,000,000

[1] Dividends available for distribution as of December 31, 2013 (relating to the dividend period for the three months ended March 31, 2014) are calculated based on our net worth as of December 31, 2013 less the applicable capital reserve amount of $2.4 billion. For quarterly dividend periods in 2013, dividends distributed were calculated based on our net worth as of the end of the immediately preceding fiscal quarter less the applicable capital reserve amount of $3.0 billion. During the years ended December 31, 2012 and 2011, an annual dividend rate of 10% on the aggregate liquidation preference was used to calculate the dividend.

[2] Includes 4.6 billion, 4.7 billion and 4.6 billion for the years ended December 31, 2013, 2012 and 2011, respectively, 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 December 31, 2013, 2012 and 2011, respectively.