Short-Term Borrowings and Long-Term Debt Short-term Debt (Details) (USD $)	Mar. 31, 2014		Dec. 31, 2013
Short-term Borrowings [Line Items]
Federal funds purchased and securities sold under agreements to repurchase	$ 25,000,000	[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	67,348,000,000		74,449,000,000
Short-term debt, Weighted-average Interest Rate	0.12%	[2]	0.13%	[2]
Secured Uncommitted Line of Credit [Member]
Short-term Borrowings [Line Items]
Line of credit maximum borrowing capacity	20,000,000,000.0		20,000,000,000
Line of credit amount outstanding	0		0
Fannie Mae [Member]
Short-term Borrowings [Line Items]
Short-term debt, outstanding	65,448,000,000		72,295,000,000
Short-term debt, Weighted-average Interest Rate	0.13%	[2]	0.13%	[2]
Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt, outstanding	65,248,000,000	[3]	71,933,000,000	[3]
Short-term debt, Weighted-average Interest Rate	0.12%	[2],[3]	0.12%	[2],[3]
Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt, outstanding	200,000,000	[4]	362,000,000	[4]
Short-term debt, Weighted-average Interest Rate	0.81%	[2],[4]	1.07%	[2],[4]
Consolidated Trusts [Member]
Short-term Borrowings [Line Items]
Short-term debt, outstanding	$ 1,900,000,000		$ 2,154,000,000
Short-term debt, Weighted-average Interest Rate	0.10%	[2]	0.09%	[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		5 days
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days		360 days
Maximum [Member] | Fannie Mae [Member] | Fixed-Rate Foreign Exchange Discount Notes [Member]
Short-term Borrowings [Line Items]
Short-term debt instrument maturity	360 days		360 days

[1] Securities sold under agreements to repurchase represent agreements to repurchase 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 issued 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.