Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,311,424	$ 3,296,298
Long-term debt, weighted-average interest rate	2.83%	2.77%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,086,460	$ 3,052,923
Long-term debt, weighted-average interest rate	2.84%	2.80%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 224,964	$ 243,375
Long-term debt, weighted-average interest rate	2.66%	2.40%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 517	$ 752
Fannie Mae [Member] \| Minimum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	1 year
Long-term debt original contractual maturity greater than 10 years	10 years	10 years
Fannie Mae [Member] \| Maximum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years	10 years
Fannie Mae [Member] \| Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 193,425	$ 206,863
Long-term debt, weighted-average interest rate	2.06%	1.95%
Fannie Mae [Member] \| Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 116,563	$ 123,541
Long-term debt, weighted-average interest rate	2.28%	2.11%
Fannie Mae [Member] \| Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 69,986	$ 75,901
Long-term debt, weighted-average interest rate	1.46%	1.41%
Fannie Mae [Member] \| Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,876	$ 7,421
Long-term debt, weighted-average interest rate	4.66%	4.84%
Fannie Mae [Member] \| Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 26,118	$ 31,328
Long-term debt, weighted-average interest rate	5.54%	4.14%
Fannie Mae [Member] \| Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 1,175	$ 8,425
Long-term debt, weighted-average interest rate	1.86%	1.36%
Fannie Mae [Member] \| Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 24,589	$ 22,527
Long-term debt, weighted-average interest rate	5.66%	5.18%
Fannie Mae [Member] \| Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 354	$ 376
Long-term debt, weighted-average interest rate	9.00%	6.36%
Fannie Mae [Member] \| Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,357	$ 5,106
Long-term debt, weighted-average interest rate	9.96%	9.93%
Fannie Mae [Member] \| Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 64	$ 78
Long-term debt, weighted-average interest rate	1.66%	1.70%

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