Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,305,588	$ 3,296,298
Long-term debt, weighted-average interest rate	2.86%	2.77%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,074,693	$ 3,052,923
Long-term debt, weighted-average interest rate	2.89%	2.80%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 230,895	$ 243,375
Long-term debt, weighted-average interest rate	2.46%	2.40%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 631	$ 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	$ 199,778	$ 206,863
Long-term debt, weighted-average interest rate	1.93%	1.95%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 118,550	$ 123,541
Long-term debt, weighted-average interest rate	2.12%	2.11%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 74,077	$ 75,901
Long-term debt, weighted-average interest rate	1.37%	1.41%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,151	$ 7,421
Long-term debt, weighted-average interest rate	4.54%	4.84%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 25,816	$ 31,328
Long-term debt, weighted-average interest rate	5.21%	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	1.26%	1.36%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 24,284	$ 22,527
Long-term debt, weighted-average interest rate	5.38%	5.18%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 357	$ 376
Long-term debt, weighted-average interest rate	6.91%	6.36%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,230	$ 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	$ 71	$ 78
Long-term debt, weighted-average interest rate	1.82%	1.70%