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Short-Term and Long-Term Debt Characteristics of Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Long-Term Debt by Year of Maturity [Abstract]    
Total long-term debt $ 3,296,298 $ 3,226,737
Consolidated Trusts [Member]    
Long-Term Debt by Year of Maturity [Abstract]    
Total long-term debt 3,052,923 2,934,635
Earlier of Contractual Maturity or Next Call Date [Member]    
Long-Term Debt by Year of Maturity [Abstract]    
Total long-term debt 3,296,298  
Earlier of Contractual Maturity or Next Call Date [Member] | Consolidated Trusts [Member]    
Long-Term Debt by Year of Maturity [Abstract]    
Total long-term debt 3,052,923  
Fannie Mae [Member]    
Long-Term Debt [Line Items]    
Face amount 277,500 328,900
Long-Term Debt by Year of Maturity [Abstract]    
2018 53,932  
2019 61,760  
2020 39,712  
2021 23,945  
2022 8,463  
Thereafter 55,563  
Total long-term debt 243,375 292,102
Net unamortized discount, fair value adjustments and other cost basis adjustments 752 1,800
Fannie Mae [Member] | Zero-Coupon Debt [Member]    
Long-Term Debt [Line Items]    
Face amount $ 39,800 $ 45,900
Effective interest rate 2.29% 2.43%
Fannie Mae [Member] | Callable [Member]    
Long-Term Debt [Line Items]    
Outstanding debt $ 72,300 $ 77,300
Fannie Mae [Member] | Earlier of Contractual Maturity or Next Call Date [Member]    
Long-Term Debt by Year of Maturity [Abstract]    
2018 100,446  
2019 45,015  
2020 25,202  
2021 14,184  
2022 7,353  
Thereafter 51,175  
Total long-term debt $ 243,375