Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Long-Term Debt [Line Items]
Outstanding	$ 3,080,801,000,000	[1]	$ 3,038,147,000,000
Weighted-average interest rate	3.18%	[2]	3.84%	[2]
Fannie Mae [Member]
Long-Term Debt [Line Items]
Outstanding	510,631,000,000	[3],[4]	585,692,000,000	[3]
Weighted-average interest rate	2.25%	[2],[3]	2.42%	[2],[3]
Net unamortized discount, fair value adjustments and other cost basis adjustments	6,000,000,000	[3]	9,200,000,000	[3]
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Outstanding	465,569,000,000		505,606,000,000
Weighted-average interest rate	2.35%	[2]	2.64%	[2]
Fannie Mae [Member] | Senior Fixed Benchmark Notes and Bonds [Member]
Long-Term Debt [Line Items]
Outstanding	251,768,000,000		277,146,000,000
Weighted-average interest rate	2.59%	[2]	2.81%	[2]
Fannie Mae [Member] | Senior Fixed Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Outstanding	172,288,000,000	[5]	176,886,000,000	[5]
Weighted-average interest rate	1.35%	[2],[5]	1.61%	[2],[5]
Fannie Mae [Member] | Senior Fixed Foreign Exchange Notes and Bonds [Member]
Long-Term Debt [Line Items]
Outstanding	694,000,000		662,000,000
Weighted-average interest rate	5.44%	[2]	5.44%	[2]
Fannie Mae [Member] | Senior Fixed Other Debt [Member]
Long-Term Debt [Line Items]
Outstanding	40,819,000,000	[1],[6]	50,912,000,000	[1],[6]
Weighted-average interest rate	4.99%	[1],[2],[6]	5.29%	[1],[2],[6]
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Outstanding	38,998,000,000		72,275,000,000
Weighted-average interest rate	0.33%	[2]	0.35%	[2]
Fannie Mae [Member] | Senior Foating Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Outstanding	38,633,000,000	[5]	71,855,000,000	[5]
Weighted-average interest rate	0.27%	[2],[5]	0.32%	[2],[5]
Fannie Mae [Member] | Senior Floating Other Debt [Member]
Long-Term Debt [Line Items]
Outstanding	365,000,000	[1],[6]	420,000,000	[1],[6]
Weighted-average interest rate	8.22%	[1],[2],[6]	8.01%	[1],[2],[6]
Fannie Mae [Member] | Subordinated Fixed [Member]
Long-Term Debt [Line Items]
Outstanding	5,719,000,000		7,811,000,000
Weighted-average interest rate	7.75%	[2]	6.88%	[2]
Fannie Mae [Member] | Subordinated Fixed Qualifying Subordinated [Member]
Long-Term Debt [Line Items]
Outstanding	2,522,000,000	[7]	4,894,000,000	[7]
Weighted-average interest rate	5.00%	[2],[7]	5.08%	[2],[7]
Fannie Mae [Member] | Subordinated Fixed Subordinated Debentures [Member]
Long-Term Debt [Line Items]
Outstanding	3,197,000,000		2,917,000,000
Weighted-average interest rate	9.92%	[2]	9.91%	[2]
Fannie Mae [Member] | Secured Borrowings [Member]
Long-Term Debt [Line Items]
Outstanding	345,000,000	[8]	0	[8]
Weighted-average interest rate	1.87%	[2],[8]	0.00%	[2],[8]
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Outstanding	$ 2,570,170,000,000	[1],[9]	$ 2,452,455,000,000	[1]
Weighted-average interest rate	3.36%	[1],[2]	4.18%	[1],[2]
Minimum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	[5]	1 year	[5]
Maximum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years	[5]	10 years	[5]

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

[2] Includes unamortized discounts, premiums, fair value adjustments and other cost basis adjustments.

[3] 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.

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

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

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

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

[8] Represents our remaining liability resulting from the transfers of financial assets from our consolidated balance sheets that did not qualify as a sale under the accounting guidance for the transfer of financial instruments.

[9] 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.