Short-Term Borrowings and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,274,792	$ 3,226,737
Long-term debt, weighted-average interest rate	2.71%	2.54%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,016,835	$ 2,934,635
Long-term debt, weighted-average interest rate	2.75%	2.57%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 257,957	$ 292,102
Long-term debt, weighted-average interest rate	2.30%	2.31%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 979	$ 1,800
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	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	$ 218,961	$ 249,013
Long-term debt, weighted-average interest rate	1.90%	2.14%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 133,022	$ 153,983
Long-term debt, weighted-average interest rate	2.01%	2.16%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 78,087	$ 82,230
Long-term debt, weighted-average interest rate	1.41%	1.40%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,852	$ 12,800
Long-term debt, weighted-average interest rate	4.83%	6.74%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 33,924	$ 38,333
Long-term debt, weighted-average interest rate	3.78%	2.48%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 11,424	$ 21,476
Long-term debt, weighted-average interest rate	1.26%	0.71%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 22,131	$ 16,511
Long-term debt, weighted-average interest rate	5.08%	4.77%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 369	$ 346
Long-term debt, weighted-average interest rate	7.44%	6.75%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,987	$ 4,645
Long-term debt, weighted-average interest rate	9.93%	9.93%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 85	$ 111
Long-term debt, weighted-average interest rate	1.52%	1.44%