Short-Term Borrowings and Long-Term Debt Characteristics of Debt (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Long-term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 3,080,801,000,000	[1]	$ 3,038,147,000,000
Fannie Mae [Member]
Long-Term Debt [Line Items]
Face amount	621,800,000,000		741,600,000,000
Long-term Debt by Year of Maturity [Abstract]
2013	103,187,000,000
2014	95,040,000,000
2015	76,591,000,000
2016	53,872,000,000
2017	90,794,000,000
Thereafter	91,147,000,000
Total long-term debt	510,631,000,000	[2],[3]	585,692,000,000	[2]
Net unamortized discount, fair value adjustments and other cost basis adjustments	6,000,000,000	[2]	9,200,000,000	[2]
Fannie Mae [Member] | Zero-Coupon Debt [Member]
Long-Term Debt [Line Items]
Face amount	120,700,000,000		165,800,000,000
Effective interest rate	0.93%		0.68%
Consolidated Trusts [Member]
Long-term Debt by Year of Maturity [Abstract]
Total long-term debt	2,570,170,000,000	[1],[4]	2,452,455,000,000	[1]
Callable [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Outstanding debt	177,800,000,000		187,900,000,000
Assuming Callable Debt Redeemed at Next Available Call Date [Member]
Long-term Debt by Year of Maturity [Abstract]
Total long-term debt	3,080,801,000,000	[1]
Assuming Callable Debt Redeemed at Next Available Call Date [Member] | Fannie Mae [Member]
Long-term Debt by Year of Maturity [Abstract]
2013	252,419,000,000
2014	111,089,000,000
2015	48,130,000,000
2016	34,077,000,000
2017	42,248,000,000
Thereafter	22,668,000,000
Total long-term debt	510,631,000,000	[3]
Assuming Callable Debt Redeemed at Next Available Call Date [Member] | Consolidated Trusts [Member]
Long-term Debt by Year of Maturity [Abstract]
Total long-term debt	$ 2,570,170,000,000	[4]

[1] Includes a portion of structured debt instruments that is reported at fair value.

[2] Reported amounts include a net unamortized discount, fair value adjustments and other cost basis adjustments of $6.0 billion and $9.2 billion as of December 31, 2012 and 2011, respectively.

[3] Reported amount includes a net unamortized discount, fair value adjustments and other cost basis adjustments of $6.0 billion.

[4] Contractual maturity of debt of consolidated trusts is not a reliable indicator of expected maturity because borrowers of the underlying loans generally have the right to prepay their obligations at any time.