Short-Term Borrowings and Long-Term Debt Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 3,138,531	$ 3,125,721
Long-term debt, weighted-average interest rate	2.88%	2.88%
Fannie Mae [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 310,402	$ 315,128
Long-term debt, weighted-average interest rate	2.40%	2.41%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments	$ 2,900	$ 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	$ 272,477	$ 278,826
Long-term debt, weighted-average interest rate	2.33%	2.39%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 154,047	$ 154,057
Long-term debt, weighted-average interest rate	2.40%	2.49%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 93,385	$ 96,997
Long-term debt, weighted-average interest rate	1.50%	1.53%
Fannie Mae [Member] | Senior fixed other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 25,045	$ 27,772
Long-term debt, weighted-average interest rate	5.08%	4.88%
Fannie Mae [Member] | Senior Floating [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 33,456	$ 31,923
Long-term debt, weighted-average interest rate	1.95%	1.58%
Fannie Mae [Member] | Senior floating medium-term notes [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 20,591	$ 20,791
Long-term debt, weighted-average interest rate	0.47%	0.27%
Fannie Mae [Member] | Senior floating Connecticut Avenue Security [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 12,470	$ 10,764
Long-term debt, weighted-average interest rate	4.22%	3.84%
Fannie Mae [Member] | Senior floating other debt [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 395	$ 368
Long-term debt, weighted-average interest rate	9.06%	10.46%
Fannie Mae [Member] | Subordinated debentures [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 4,328	$ 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	$ 141	$ 152
Long-term debt, weighted-average interest rate	1.44%	1.47%
Consolidated Trusts [Member]
Long-Term Debt [Line Items]
Long-term debt, outstanding	$ 2,828,129	$ 2,810,593
Long-term debt, weighted-average interest rate	2.94%	2.94%