Short-Term Borrowings and Long-Term Debt Short-term Debt (Details) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Short-term Borrowings [Line Items]
Short-term debt	$ 108,868,000,000		$ 151,725,000,000
Short-term debt, Weighted-average Interest Rate	0.16%	[1]	0.13%	[1]
Line of credit amount outstanding	0
Secured Uncommitted Line of Credit [Member]
Short-term Borrowings [Line Items]
Line of credit maximum borrowing capacity	25,000,000,000		25,000,000,000
Unsecured Uncommitted Line of Credit [Member]
Short-term Borrowings [Line Items]
Line of credit maximum borrowing capacity	500,000,000		500,000,000
Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt	105,062,000,000		146,752,000,000
Short-term debt, Weighted-average Interest Rate	0.16%	[1]	0.13%	[1]
Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt	104,547,000,000	[2]	146,301,000,000	[2]
Short-term debt, Weighted-average Interest Rate	0.15%	[1],[2]	0.13%	[1],[2]
Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt	515,000,000	[3]	371,000,000	[3]
Short-term debt, Weighted-average Interest Rate	1.71%	[1],[3]	1.88%	[1],[3]
Fannie Mae [Member] | Fixed-Rate Other Debt [Member]
Short-term Borrowings [Line Items]
Short-term debt	0	[4]	80,000,000	[4]
Short-term debt, Weighted-average Interest Rate	0.00%	[1],[4]	0.04%	[1],[4]
Consolidated Trusts [Member]
Short-term Borrowings [Line Items]
Short-term debt	$ 3,806,000,000		$ 4,973,000,000
Short-term debt, Weighted-average Interest Rate	0.15%	[1]	0.09%	[1]
Minimum [Member] | Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	0 days	[2]	0 days	[2]
Minimum [Member] | Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	5 days	[3]	5 days	[3]
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days	[2]	360 days	[2]
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days	[3]	360 days	[3]

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

[2] Represents unsecured general obligations with maturities ranging from overnight to 360 days from the date of issuance.

[3] Represents foreign exchange discount notes we issue in the Euro commercial paper market with maturities ranging from 5 to 360 days which enable investors to hold short-term investments in different currencies. We do not incur foreign exchange risk on these transactions, as we simultaneously enter into foreign currency swaps that have the effect of converting debt that we issue in foreign denominated currencies into U.S. dollars.

[4] Includes foreign exchange discount notes denominated in U.S. dollars.