Short-Term Borrowings and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,256,655	$ 3,226,737
Long-term debt, weighted-average interest rate	2.73%	2.54%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 272,619	$ 292,102
Long-term debt, weighted-average interest rate	2.20%	2.31%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 1,000	$ 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	$ 227,650	$ 249,013
Long-term debt, weighted-average interest rate	1.87%	2.14%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 137,509	$ 153,983
Long-term debt, weighted-average interest rate	1.96%	2.16%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 82,215	$ 82,230
Long-term debt, weighted-average interest rate	1.42%	1.40%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 7,926	$ 12,800
Long-term debt, weighted-average interest rate	4.82%	6.74%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 40,005	$ 38,333
Long-term debt, weighted-average interest rate	3.14%	2.48%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 19,051	$ 21,476
Long-term debt, weighted-average interest rate	1.11%	0.71%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 20,589	$ 16,511
Long-term debt, weighted-average interest rate	5.03%	4.77%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 365	$ 346
Long-term debt, weighted-average interest rate	7.20%	6.75%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,870	$ 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	$ 94	$ 111
Long-term debt, weighted-average interest rate	1.60%	1.44%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,984,036	$ 2,934,635
Long-term debt, weighted-average interest rate	2.78%	2.57%