Short-Term Borrowings and Long-Term Debt Short-term Debt (Details) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Short-term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase, Outstanding	$ 218,000,000	[1]	$ 0	[1]
Federal funds purchased and securities sold under agreements to repurchase, weighted average interest rate	0.00%	[1],[2]	0.00%	[1],[2]
Short-term debt, Outstanding	118,294,000,000		108,716,000,000
Short-term debt, Weighted-average Interest Rate	0.15%	[2]	0.16%	[2]
Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt, Outstanding	115,285,000,000		105,233,000,000
Short-term debt, Weighted-average Interest Rate	0.15%	[2]	0.16%	[2]
Consolidated Trusts [Member]
Short-term Borrowings [Line Items]
Short-term debt, Outstanding	3,009,000,000		3,483,000,000
Short-term debt, Weighted-average Interest Rate	0.14%	[2]	0.15%	[2]
Fixed-Rate Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt, Outstanding	114,815,000,000	[3]	104,730,000,000	[3]
Short-term debt, Weighted-average Interest Rate	0.14%	[2],[3]	0.15%	[2],[3]
Fixed-Rate Foreign Exchange Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt, Outstanding	470,000,000	[4]	503,000,000	[4]
Short-term debt, Weighted-average Interest Rate	1.57%	[2],[4]	1.61%	[2],[4]
Secured Uncommitted Line of Credit [Member]
Short-term Borrowings [Line Items]
Line of credit maximum borrowing capacity	15,000,000,000		15,000,000,000
Line of credit amount outstanding	0		0
Unsecured Uncommitted Line of Credit [Member]
Short-term Borrowings [Line Items]
Line of credit maximum borrowing capacity	$ 0		$ 0
Minimum [Member] | Fixed-Rate Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	0 days		0 days
Minimum [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	5 days		5 days
Maximum [Member] | Fixed-Rate Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days		360 days
Maximum [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member] | Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days		360 days

[1] Securities sold under agreements to repurchase represents agreements to repurchase securities for a specified price, with repayment generally occurring on the following day.

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

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

[4] 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.