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Regulatory Capital Requirements (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Jan. 01, 2018
Dec. 31, 2017
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Regulatory Capital Requirements [Abstract]              
Component of statutory minimum capital requirement, percentage of the unpaid principal balance of outstanding Fannie Mae MBS held by third parties 0.45%       0.45%    
Component of critical capital requirement, percentage of the unpaid principal balance of outstanding Fannie Mae MBS held by third parties 0.25%            
Number of days of that liabilities exceed assets requiring FHFA to place us into receivership 60 days            
Net worth $ 6,071     $ 4,200 $ 4,059 $ 3,720 $ 9,591
Core capital (111,836)       (114,526)    
Statutory minimum capital requirement 24,351       25,144    
Deficit of core capital over statutory minimum capital requirement $ (136,187)       $ (139,670)    
Component of statutory minimum capital requirement, percentage of on-balance sheet assets, except those underlying Fannie Mae MBS held by third parties 2.50%       2.50%    
Component of statutory minimum capital requirement, maximum percentage of other off-balance sheet obligations 0.45%       0.45%    
Component of critical capital requirement, percentage of on-balance sheet assets, except those underlying Fannie Mae MBS held by third parties 1.25%            
Component of critical capital requirement, percentage of other off-balance sheet obligations 0.25%            
US Treasury [Member]              
Related Parties [Line Items]              
Capital reserve amount for the fiscal year based on the amended Senior Preferred Stock Purchase agreement $ 1,200       $ 1,800    
US Treasury [Member] | Scenario, Forecast [Member]              
Related Parties [Line Items]              
Capital reserve amount for the fiscal year based on the amended Senior Preferred Stock Purchase agreement   $ 0 $ 600