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Short-Term Borrowings and Long-Term Debt Long-Term Debt (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 3,243,645 $ 3,226,737
Long-term debt, weighted-average interest rate 2.82% 2.54%
Fannie Mae [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 289,731 $ 292,102
Long-term debt, weighted-average interest rate 2.34% 2.31%
Unamortized discounts and premiums, other cost basis adjustments and fair value adjustments $ 1,400 $ 1,800
Fannie Mae [Member] | Minimum [Member]    
Long-Term Debt [Line Items]    
Medium-term notes original contractual maturity 1 year 1 year
Long-term debt, original contractual maturity greater than 10 years 10 years 10 years
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 $ 247,193 $ 249,013
Long-term debt, weighted-average interest rate 2.11% 2.14%
Fannie Mae [Member] | Senior fixed benchmark notes and bonds [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 150,996 $ 153,983
Long-term debt, weighted-average interest rate 2.09% 2.16%
Fannie Mae [Member] | Senior fixed medium-term notes [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 83,310 $ 82,230
Long-term debt, weighted-average interest rate 1.42% 1.40%
Fannie Mae [Member] | Senior fixed other debt [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 12,887 $ 12,800
Long-term debt, weighted-average interest rate 6.78% 6.74%
Fannie Mae [Member] | Senior floating [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 37,680 $ 38,333
Long-term debt, weighted-average interest rate 2.91% 2.48%
Fannie Mae [Member] | Senior floating medium-term notes [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 18,751 $ 21,476
Long-term debt, weighted-average interest rate 0.91% 0.71%
Fannie Mae [Member] | Senior floating Connecticut Avenue Securities [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 18,579 $ 16,511
Long-term debt, weighted-average interest rate 4.93% 4.77%
Fannie Mae [Member] | Senior floating other debt [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 350 $ 346
Long-term debt, weighted-average interest rate 7.26% 6.75%
Fannie Mae [Member] | Subordinated debentures [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 4,756 $ 4,645
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 $ 102 $ 111
Long-term debt, weighted-average interest rate 1.59% 1.44%
Consolidated Trusts [Member]    
Long-Term Debt [Line Items]    
Long-term debt, outstanding $ 2,953,914 $ 2,934,635
Long-term debt, weighted-average interest rate 2.86% 2.57%