Short-Term Borrowings and Long-Term Debt Debt Called and Repurchased (Details) (Fannie Mae [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Called [Member]
Extinguishment of Debt [Line Items]
Amount of debt called or repurchased	$ 200,713	$ 201,651	$ 289,770
Weighted-average interest rate	2.00%	2.40%	3.10%
Debt Repurchased [Member]
Extinguishment of Debt [Line Items]
Amount of debt called or repurchased	$ 44	$ 2,887	$ 1,333
Weighted-average interest rate	3.80%	3.10%	3.30%