Income Taxes - Narratives (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 US Treasury [Member]	Dec. 31, 2009 US Treasury [Member]
Income Tax Contingency [Line Items]
Valuation allowance	$ 58,851,000,000	$ 64,080,000,000
Period of cumulative pre-tax loss position	3 years
Total available funding from US Treasury pursuant to the senior preferred stock agreement after 2012, if valuation allowance released			84,000,000,000
Total available funding from US Treasury pursuant to the senior preferred stock agreement after 2012			117,600,000,000	124,800,000,000
Reduction in the funds available to us under the senior preferred stock purchase agreement			$ 34,000,000,000

Represents total available funding from U.S. Treasury pursuant to the senior preferred stock agreement after 2012 if we do not have a positive net worth as of December 31, 2012. In the event we have a positive net worth as of December 31, 2012, then the amount of funding available after 2012 under the amended senior preferred stock purchase agreement will depend on the size of that positive net worth relative to the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, as follows: 1) If our positive net worth as of December 31, 2012 is less than the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, then the amount of available funding will be $124.8 billion less our positive net worth as of December 31, 2012. 2) If our positive net worth as of December 31, 2012 is greater than the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, then the amount of available funding will be $124.8 billion less the cumulative draws attributable to periods during 2010, 2011, and 2012.

No definition available.

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