Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,367,024	$ 3,296,298
Long-term debt, weighted-average interest rate	3.02%	2.77%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,159,846	$ 3,052,923
Long-term debt, weighted-average interest rate	3.03%	2.80%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 207,178	$ 243,375
Long-term debt, weighted-average interest rate	2.83%	2.40%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 413	$ 752
Fannie Mae [Member] \| Minimum [Member]
Long-Term Debt [Line Items]
Medium-term Notes Original Contractual Maturity 1	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 1	10 years	10 years
Fannie Mae [Member] \| Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 171,344	$ 206,863
Long-term debt, weighted-average interest rate	2.13%	1.95%
Fannie Mae [Member] \| Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 103,206	$ 123,541
Long-term debt, weighted-average interest rate	2.36%	2.11%
Fannie Mae [Member] \| Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 61,455	$ 75,901
Long-term debt, weighted-average interest rate	1.48%	1.41%
Fannie Mae [Member] \| Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,683	$ 7,421
Long-term debt, weighted-average interest rate	4.62%	4.84%
Fannie Mae [Member] \| Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 30,166	$ 31,328
Long-term debt, weighted-average interest rate	5.52%	4.14%
Fannie Mae [Member] \| Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,174	$ 8,425
Long-term debt, weighted-average interest rate	2.36%	1.36%
Fannie Mae [Member] \| Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 25,641	$ 22,527
Long-term debt, weighted-average interest rate	5.97%	5.18%
Fannie Mae [Member] \| Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 351	$ 376
Long-term debt, weighted-average interest rate	10.19%	6.36%
Fannie Mae [Member] \| Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,617	$ 5,106
Long-term debt, weighted-average interest rate	9.64%	9.93%
Fannie Mae [Member] \| Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 51	$ 78
Long-term debt, weighted-average interest rate	1.96%	1.70%

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