Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Long-Term Debt [Line Items]
Outstanding	$ 3,087,842,000,000		$ 3,038,147,000,000
Weighted-average interest rate	3.32%	[1]	3.84%	[1]
Fannie Mae [Member]
Long-Term Debt [Line Items]
Outstanding	547,909,000,000	[2]	585,692,000,000	[2]
Weighted-average interest rate	2.16%	[1],[2]	2.42%	[1],[2]
Net discount and other cost basis adjustments	6,200,000,000	[2]	9,200,000,000	[2]
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Outstanding	477,984,000,000		505,606,000,000
Weighted-average interest rate	2.34%	[1]	2.64%	[1]
Fannie Mae [Member] | Senior Fixed Benchmark Notes and Bonds [Member]
Long-Term Debt [Line Items]
Outstanding	260,774,000,000		277,146,000,000
Weighted-average interest rate	2.58%	[1]	2.81%	[1]
Fannie Mae [Member] | Senior Fixed Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Outstanding	174,867,000,000	[3]	176,886,000,000	[3]
Weighted-average interest rate	1.31%	[1],[3]	1.61%	[1],[3]
Fannie Mae [Member] | Senior Fixed Foreign Exchange Notes and Bonds [Member]
Long-Term Debt [Line Items]
Outstanding	689,000,000		662,000,000
Weighted-average interest rate	5.30%	[1]	5.44%	[1]
Fannie Mae [Member] | Senior Fixed Other Debt [Member]
Long-Term Debt [Line Items]
Outstanding	41,654,000,000	[4],[5]	50,912,000,000	[4],[5]
Weighted-average interest rate	5.11%	[1],[4],[5]	5.29%	[1],[4],[5]
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Outstanding	63,912,000,000		72,275,000,000
Weighted-average interest rate	0.33%	[1]	0.35%	[1]
Fannie Mae [Member] | Senior Foating Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Outstanding	63,516,000,000	[3]	71,855,000,000	[3]
Weighted-average interest rate	0.29%	[1],[3]	0.32%	[1],[3]
Fannie Mae [Member] | Senior Floating Other Debt [Member]
Long-Term Debt [Line Items]
Outstanding	396,000,000	[4],[5]	420,000,000	[4],[5]
Weighted-average interest rate	7.46%	[1],[4],[5]	8.01%	[1],[4],[5]
Fannie Mae [Member] | Subordinated Fixed [Member]
Long-Term Debt [Line Items]
Outstanding	5,646,000,000		7,811,000,000
Weighted-average interest rate	7.72%	[1]	6.88%	[1]
Fannie Mae [Member] | Subordinated Fixed Qualifying Subordinated [Member]
Long-Term Debt [Line Items]
Outstanding	2,522,000,000	[6]	4,894,000,000	[6]
Weighted-average interest rate	5.00%	[1],[6]	5.08%	[1],[6]
Fannie Mae [Member] | Subordinated Fixed Subordinated Debentures [Member]
Long-Term Debt [Line Items]
Outstanding	3,124,000,000		2,917,000,000
Weighted-average interest rate	9.92%	[1]	9.91%	[1]
Fannie Mae [Member] | Secured Borrowings [Member]
Long-Term Debt [Line Items]
Outstanding	367,000,000	[7]	0	[7]
Weighted-average interest rate	1.87%	[1],[7]	0.00%	[1],[7]
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Outstanding	$ 2,539,933,000,000	[4]	$ 2,452,455,000,000	[4]
Weighted-average interest rate	3.58%	[1],[4]	4.18%	[1],[4]
Minimum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	[3]	1 year	[3]
Maximum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years	[3]	10 years	[3]

[1] Includes the effects of discounts, premiums and other cost basis adjustments.

[2] Reported amounts include a net discount and other cost basis adjustments of $6.2 billion and $9.2 billion as of September 30, 2012 and December 31, 2011, respectively.

[3] Includes long-term debt with an original contractual maturity of greater than 1 year and up to 10 years, excluding zero-coupon debt.

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

[5] Includes long-term debt that is not included in other debt categories.

[6] Consists of subordinated debt issued with an interest deferral feature.

[7] Represents remaining liability for transfer of financial assets from our condensed consolidated balance sheets that did not qualify as a sale.