Short-Term Borrowings and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,181,059	$ 3,125,721
Long-term debt, weighted-average interest rate	2.51%	2.88%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 300,126	$ 315,128
Long-term debt, weighted-average interest rate	2.28%	2.41%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 2,000	$ 3,200
Fannie Mae [Member] | Minimum [Member]
Long-Term Debt [Line Items]
Medium-term notes original contractual maturity	1 year	1 year
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	$ 250,699	$ 278,826
Long-term debt, weighted-average interest rate	2.19%	2.39%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 153,481	$ 154,057
Long-term debt, weighted-average interest rate	2.23%	2.49%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 83,661	$ 96,997
Long-term debt, weighted-average interest rate	1.39%	1.53%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 13,557	$ 27,772
Long-term debt, weighted-average interest rate	6.58%	4.88%
Fannie Mae [Member] | Senior floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 44,770	$ 31,923
Long-term debt, weighted-average interest rate	2.01%	1.58%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 28,715	$ 20,791
Long-term debt, weighted-average interest rate	0.59%	0.27%
Fannie Mae [Member] | Senior floating Connecticut Avenue Security [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 15,636	$ 10,764
Long-term debt, weighted-average interest rate	4.59%	3.84%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 419	$ 368
Long-term debt, weighted-average interest rate	6.65%	10.46%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,537	$ 4,227
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	$ 120	$ 152
Long-term debt, weighted-average interest rate	1.42%	1.47%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,880,933	$ 2,810,593
Long-term debt, weighted-average interest rate	2.53%	2.94%