Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) (USD $)	6 Months Ended		12 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,096,775,000,000		$ 3,160,074,000,000
Long-term debt, weighted-average interest rate	3.05%	[1]	3.10%	[1]
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	386,609,000,000	[2]	457,139,000,000	[2]
Long-term debt, weighted-average interest rate	2.21%	[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	347,780,000,000		412,805,000,000
Long-term debt, weighted-average interest rate	2.31%	[1]	2.24%	[1]
Fannie Mae [Member] | Senior Fixed Benchmark Notes and Bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	186,556,000,000		212,234,000,000
Long-term debt, weighted-average interest rate	2.40%	[1]	2.45%	[1]
Fannie Mae [Member] | Senior Fixed Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	124,483,000,000	[3]	161,445,000,000	[3]
Long-term debt, weighted-average interest rate	1.36%	[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	680,000,000		682,000,000
Long-term debt, weighted-average interest rate	5.25%	[1]	5.41%	[1]
Fannie Mae [Member] | Senior Fixed Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	36,061,000,000	[4]	38,444,000,000	[4]
Long-term debt, weighted-average interest rate	5.02%	[1],[4]	4.99%	[1],[4]
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	34,925,000,000		39,396,000,000
Long-term debt, weighted-average interest rate	0.46%	[1]	0.32%	[1]
Fannie Mae [Member] | Senior Floating Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	31,493,000,000	[3]	38,441,000,000	[3]
Long-term debt, weighted-average interest rate	0.18%	[1],[3]	0.20%	[1],[3]
Fannie Mae [Member] | Senior Floating Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,432,000,000	[4],[5]	955,000,000	[4],[5]
Long-term debt, weighted-average interest rate	3.18%	[1],[4],[5]	5.18%	[1],[4],[5]
Fannie Mae [Member] | Subordinated Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,674,000,000		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	3,674,000,000		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	230,000,000	[6]	262,000,000	[6]
Long-term debt, weighted-average interest rate	1.88%	[1],[6]	1.86%	[1],[6]
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,710,166,000,000	[4]	$ 2,702,935,000,000	[4]
Long-term debt, weighted-average interest rate	3.17%	[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 June 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.