Long-term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,125,721	$ 3,115,583
Long-term debt, weighted-average interest rate	2.88%	2.93%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 315,128	$ 355,431
Long-term debt, weighted-average interest rate	2.41%	2.24%
Net unamortized discount, fair value adjustments and other cost basis adjustments	$ 3,200	$ 4,100
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 278,826	$ 320,507
Long-term debt, weighted-average interest rate	2.39%	2.29%
Fannie Mae [Member] | Senior Fixed Benchmark Notes and Bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 154,057	$ 173,010
Long-term debt, weighted-average interest rate	2.49%	2.41%
Fannie Mae [Member] | Senior Fixed Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 96,997	$ 114,556
Long-term debt, weighted-average interest rate	1.53%	1.42%
Fannie Mae [Member] | Senior Fixed Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 27,772	$ 32,941
Long-term debt, weighted-average interest rate	4.88%	4.65%
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 31,923	$ 30,873
Long-term debt, weighted-average interest rate	1.58%	0.81%
Fannie Mae [Member] | Senior Floating Medium-Term Notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 20,791	$ 24,469
Long-term debt, weighted-average interest rate	0.27%	0.15%
Fannie Mae [Member] | Senior Floating Connecticut Avenue Security [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 10,764	$ 6,041
Long-term debt, weighted-average interest rate	3.84%	2.97%
Fannie Mae [Member] | Senior Floating Other Debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 368	$ 363
Long-term debt, weighted-average interest rate	10.46%	8.71%
Fannie Mae [Member] | Subordinated Debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,227	$ 3,849
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	$ 152	$ 202
Long-term debt, weighted-average interest rate	1.47%	1.90%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,810,593	$ 2,760,152
Long-term debt, weighted-average interest rate	2.94%	3.02%
Minimum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	1 year
Maximum [Member] | Fannie Mae [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	10 years	10 years