Short-Term and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,371,939	$ 3,367,024
Long-term debt, weighted-average interest rate	3.06%	3.02%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,173,772	$ 3,159,846
Long-term debt, weighted-average interest rate	3.06%	3.03%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 198,167	$ 207,178
Long-term debt, weighted-average interest rate	2.93%	2.83%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 213	$ 413
Fannie Mae [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year
Long-term debt original contractual maturity greater than 10 years	10 years
Fannie Mae [Member] | Maximum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 161,605	$ 171,344
Long-term debt, weighted-average interest rate	2.20%	2.13%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 97,710	$ 103,206
Long-term debt, weighted-average interest rate	2.46%	2.36%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 57,220	$ 61,455
Long-term debt, weighted-average interest rate	1.48%	1.48%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 6,675	$ 6,683
Long-term debt, weighted-average interest rate	4.61%	4.62%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 31,014	$ 30,166
Long-term debt, weighted-average interest rate	5.50%	5.52%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,574	$ 4,174
Long-term debt, weighted-average interest rate	2.47%	2.36%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 25,064	$ 25,641
Long-term debt, weighted-average interest rate	6.13%	5.97%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 376	$ 351
Long-term debt, weighted-average interest rate	7.85%	10.19%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 5,502	$ 5,617
Long-term debt, weighted-average interest rate	9.67%	9.64%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 46	$ 51
Long-term debt, weighted-average interest rate	2.06%	1.96%