Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,115,822	$ 3,115,583
Long-term debt, weighted-average interest rate	2.79%	2.93%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 343,747	$ 355,431
Long-term debt, weighted-average interest rate	2.31%	2.24%
Unamortized discount and premiums, other cost basis adjustments and fair value adjustments	$ 3,500	$ 4,100
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 312,256	$ 320,507
Long-term debt, weighted-average interest rate	2.31%	2.29%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 170,531	$ 173,010
Long-term debt, weighted-average interest rate	2.42%	2.41%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 111,556	$ 114,556
Long-term debt, weighted-average interest rate	1.49%	1.42%
Fannie Mae [Member] | Senior fixed foreign exchange notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 624	$ 619
Long-term debt, weighted-average interest rate	5.29%	5.44%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 29,545	$ 32,322
Long-term debt, weighted-average interest rate	4.74%	4.63%
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 27,280	$ 30,873
Long-term debt, weighted-average interest rate	1.22%	0.81%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 18,420	$ 24,469
Long-term debt, weighted-average interest rate	0.19%	0.15%
Fannie Mae [Member] | Senior floating Connecticut Avenue Security [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 8,514	$ 6,041
Long-term debt, weighted-average interest rate	3.19%	2.97%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 346	$ 363
Long-term debt, weighted-average interest rate	8.64%	8.71%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,034	$ 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	$ 177	$ 202
Long-term debt, weighted-average interest rate	1.92%	1.90%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,772,075	$ 2,760,152
Long-term debt, weighted-average interest rate	2.85%	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