Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,102,277,000,000		$ 3,160,074,000,000
Long-term debt, weighted-average interest rate	2.99%	[1]	3.10%	[1]
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	377,553,000,000	[2]	457,139,000,000	[2]
Long-term debt, weighted-average interest rate	2.22%	[1],[2]	2.14%	[1],[2]
Unamortized discount, fair value adjustments and other cost basis adjustments	4,100,000,000		4,800,000,000
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	346,999,000,000		412,805,000,000
Long-term debt, weighted-average interest rate	2.25%	[1]	2.24%	[1]
Fannie Mae [Member] | Senior Fixed Benchmark Notes and Bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	188,002,000,000		212,234,000,000
Long-term debt, weighted-average interest rate	2.41%	[1]	2.45%	[1]
Fannie Mae [Member] | Senior Fixed Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	125,106,000,000	[3]	161,445,000,000	[3]
Long-term debt, weighted-average interest rate	1.38%	[1],[3]	1.28%	[1],[3]
Fannie Mae [Member] | Senior Fixed Foreign Exchange Notes and Bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	644,000,000		682,000,000
Long-term debt, weighted-average interest rate	5.23%	[1]	5.41%	[1]
Fannie Mae [Member] | Senior Fixed Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	33,247,000,000		38,444,000,000	[4]
Long-term debt, weighted-average interest rate	4.61%	[1]	4.99%	[1]
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	26,578,000,000		39,396,000,000
Long-term debt, weighted-average interest rate	0.70%	[1]	0.32%	[1]
Fannie Mae [Member] | Senior Floating Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	21,374,000,000	[3]	38,441,000,000	[3]
Long-term debt, weighted-average interest rate	0.17%	[1],[3]	0.20%	[1],[3]
Fannie Mae [Member] | Senior Floating Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	5,204,000,000	[4],[5]	955,000,000	[4],[5]
Long-term debt, weighted-average interest rate	2.90%	[1],[4],[5]	5.18%	[1],[4],[5]
Fannie Mae [Member] | Subordinated Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,760,000,000		4,676,000,000
Long-term debt, weighted-average interest rate	9.92%	[1]	8.76%	[1]
Fannie Mae [Member] | Subordinated Fixed Qualifying Subordinated [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	0		1,169,000,000
Long-term debt, weighted-average interest rate	0.00%	[1]	5.27%	[1]
Fannie Mae [Member] | Subordinated Fixed Subordinated Debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,760,000,000		3,507,000,000
Long-term debt, weighted-average interest rate	9.92%	[1]	9.92%	[1]
Fannie Mae [Member] | Secured Borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	216,000,000	[6]	262,000,000	[6]
Long-term debt, weighted-average interest rate	1.89%	[1],[6]	1.86%	[1],[6]
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,724,724,000,000	[4]	$ 2,702,935,000,000	[4]
Long-term debt, weighted-average interest rate	3.10%	[1],[4]	3.26%	[1],[4]
Minimum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term Notes Original Contractual Maturity	1 year		1 year
Maximum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term Notes Original Contractual Maturity	10 years		10 years

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

[2] Reported amounts include unamortized discounts and premiums, other cost basis adjustments and fair value adjustments of $4.1 billion and $4.8 billion as of September 30, 2014 and December 31, 2013, 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 credit risk sharing securities issued under our Connecticut Avenue Securities series, which transfers some of the credit risk of our mortgage loans to the investors in these securities.

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