Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,426,046	$ 3,367,024
Long-term debt, weighted-average interest rate	2.81%	3.02%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,248,336	$ 3,159,846
Long-term debt, weighted-average interest rate	2.79%	3.03%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 177,710	$ 207,178
Long-term debt, weighted-average interest rate	3.14%	2.83%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 30	$ 413
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	$ 138,718	$ 171,344
Long-term debt, weighted-average interest rate	2.44%	2.13%
Fannie Mae [Member] \| Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 91,603	$ 103,206
Long-term debt, weighted-average interest rate	2.66%	2.36%
Fannie Mae [Member] \| Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 40,973	$ 61,455
Long-term debt, weighted-average interest rate	1.60%	1.48%
Fannie Mae [Member] \| Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,142	$ 6,683
Long-term debt, weighted-average interest rate	4.84%	4.62%
Fannie Mae [Member] \| Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 33,172	$ 30,166
Long-term debt, weighted-average interest rate	4.86%	5.52%
Fannie Mae [Member] \| Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 9,773	$ 4,174
Long-term debt, weighted-average interest rate	2.28%	2.36%
Fannie Mae [Member] \| Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 22,977	$ 25,641
Long-term debt, weighted-average interest rate	5.90%	5.97%
Fannie Mae [Member] \| Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 422	$ 351
Long-term debt, weighted-average interest rate	7.79%	10.19%
Fannie Mae [Member] \| Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,783	$ 5,617
Long-term debt, weighted-average interest rate	9.99%	9.64%
Fannie Mae [Member] \| Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 37	$ 51
Long-term debt, weighted-average interest rate	2.24%	1.96%

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