Short-Term Borrowings and Long-Term Debt Long-term Debt (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,111,819,000,000	3,115,583,000,000
Long-term debt, weighted-average interest rate	2.84%	2.93%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	349,391,000,000	355,431,000,000
Long-term debt, weighted-average interest rate	2.29%	2.24%
Unamortized discount, fair value adjustments and other cost basis adjustments	3,600,000,000	4,100,000,000
Fannie Mae [Member] | Senior Fixed [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	318,922,000,000	320,507,000,000
Long-term debt, weighted-average interest rate	2.29%	2.29%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	170,531,000,000	173,010,000,000
Long-term debt, weighted-average interest rate	2.44%	2.41%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	116,246,000,000	114,556,000,000
Long-term debt, weighted-average interest rate	1.44%	1.42%
Fannie Mae [Member] | Senior fixed foreign exchange notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	590,000,000	619,000,000
Long-term debt, weighted-average interest rate	5.37%	5.44%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	31,555,000,000	32,322,000,000
Long-term debt, weighted-average interest rate	4.57%	4.63%
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	26,340,000,000	30,873,000,000
Long-term debt, weighted-average interest rate	1.10%	0.81%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	18,419,000,000	24,469,000,000
Long-term debt, weighted-average interest rate	0.18%	0.15%
Fannie Mae [Member] | Senior floating Connecticut Avenue Security [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	7,530,000,000	6,041,000,000
Long-term debt, weighted-average interest rate	3.10%	2.97%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	391,000,000	363,000,000
Long-term debt, weighted-average interest rate	7.89%	8.71%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	3,940,000,000	3,849,000,000
Long-term debt, weighted-average interest rate	9.84%	9.93%
Fannie Mae [Member] | Secured borrowings [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	189,000,000	202,000,000
Long-term debt, weighted-average interest rate	1.91%	1.90%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	2,762,428,000,000	2,760,152,000,000
Long-term debt, weighted-average interest rate	2.91%	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