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Regulatory Capital Requirements (Details) - USD ($)
$ in Millions
Jan. 01, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
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%   0.25%    
Net worth (deficit)   $ (3,686) $ 3,600 $ 6,071 $ 4,059 $ 3,720
Core capital   (121,389)   (111,836)    
Statutory minimum capital requirement   23,007   24,351    
Deficit of core capital over statutory minimum capital requirement   $ (144,396)   $ (136,187)    
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%   1.25%    
Component of critical capital requirement, percentage of other off-balance sheet obligations   0.25%   0.25%    
US Treasury [Member] | Scenario, Forecast [Member]            
Related Parties [Line Items]            
Capital Reserve Amount, Fiscal Year, Senior Preferred Stock Purchase Agreement, Amendment $ 3,000