Investments in Securities Other-than-temporary Impairments 1 (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Aggregate losses in continuous unrealized loss position	$ 660	$ 2,006
Fair value of securities with unrealized losses for 12 consecutive months or longer as percentage of amortized cost basis	85.00%

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

No definition available.

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Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant