Earnings (Loss) Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income attributable to Fannie Mae	$ 1,888us-gaap_NetIncomeLoss	$ 5,325us-gaap_NetIncomeLoss
Dividends available for distribution to senior preferred stockholder	(1,796)	(5,692)
Net income (loss) attributable to common stockholders	$ 92	$ (367)
Weighted-average common shares outstanding, Basic	5,762,000,000	5,762,000,000
Convertible Preferred Stock	131,000,000	0
Weighted-average common of shares outstanding, Diluted	5,893,000,000	5,762,000,000
Earnings (loss) per share, Basic (in dollars per share)	$ 0.02us-gaap_EarningsPerShareBasic	$ (0.06)us-gaap_EarningsPerShareBasic
Earnings (loss) per share, Diluted (in dollars per share)	$ 0.02us-gaap_EarningsPerShareDiluted	$ (0.06)us-gaap_EarningsPerShareDiluted
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

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible preferred stock using the if-converted method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The aggregate value of preferred stock dividends and other adjustments necessary to derive net income apportioned to common stockholders.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Shares issuable for little or no cash consideration upon the satisfaction of certain conditions (contingently issuable shares) are considered outstanding common shares and included in the computation of basic Earnings Per Share as of the date that all necessary conditions have been satisfied (in essence, when issuance of the shares is no longer contingent). Outstanding common shares that are contingently returnable (that is, subject to recall) are treated in the same manner as contingently issuable shares. Contingently issuable shares include shares that (a) will be issued in the future upon the satisfaction of specified conditions, (b) have been placed in escrow and all or part must be returned if specified conditions are not met, or (c) have been issued but the holder must return all or part if specified conditions are not met. The number of contingently issuable shares is determined by relating the portion of time within a reporting period that these shares have been outstanding to the total time in that period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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