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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 6 Months Ended 36 Months Ended 84 Months Ended 134 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2010
Dec. 31, 2017
Jun. 30, 2018
Jul. 27, 2018
May 03, 2007
Equity And Earnings Per Share [Line Items]                  
IBG Holdings ownership % of IBG LLC 82.20%   82.20%       82.20%   90.00%
IBG Holdings Redemption of IBG LLC                 10.00%
IBG Holdings LLC Ownership Percentage of Class B Common Stock                 100.00%
Preferred stock shares authorized 10,000   10,000     10,000 10,000    
Preferred stock shares issued 0   0     0 0    
Preferred stock shares outstanding 0   0     0 0    
Amortization period DTA (years)     15 years            
Unamortized deferred tax asset arising from equity offerings $ 137   $ 137     $ 146 $ 137    
Percent of tax savings owed to IBG Holdings LLC     85.00%            
Percentage of tax savings retained by IBG Inc.     15.00%            
Deferred tax asset from common stock offerings 483   $ 483       483    
Tax savings owed to IBG Holdings LLC 410   410       410    
Tax savings retained by IBG Inc. $ 73   $ 73       73    
Tax savings paid to IBG Holdings LLC             $ 159    
Shares reserved for future issuance                 360,000,000
Shares redeemed by IBG Holdings from IBG LLC         5,013,259        
Cash Redemptions IBG Holdings         $ 114        
Fair Value of Issued Shares in Exchange for Membership Interests           $ 410      
Shares Issued           13,858,355      
Thomas Peterffy and Affiliates Ownership 89.20%   89.20%       89.20%   84.60%
Distribution from IBG LLC     $ 303            
Cash distribution to IBG, Inc.     $ 53            
Dividend per share     $ 0.10            
Dividends paid to common shareholders $ 7 $ 7 $ 14 $ 14          
Declaration Date     Jul. 17, 2018            
Payment Date     Sep. 14, 2018            
Record Date     Aug. 31, 2018            
IBG LLC [Member]                  
Equity And Earnings Per Share [Line Items]                  
IBG Inc. ownership % of IBG LLC 17.80%   17.80%       17.80%   10.00%
IBG LLC [Member] | Subsequent Event [Member]                  
Equity And Earnings Per Share [Line Items]                  
IBG Inc. ownership % of IBG LLC               18.10%  
Common stock, shares issued               1,537,727  
Common Class A                  
Equity And Earnings Per Share [Line Items]                  
Common stock, par value $ 0.01   $ 0.01     $ 0.01 $ 0.01    
Shares authorized 1,000,000,000   1,000,000,000     1,000,000,000 1,000,000,000    
Common stock, shares issued 73,674,391   73,674,391     71,609,049 73,674,391    
Common stock, shares outstanding 73,544,943   73,544,943     71,475,755 73,544,943    
Common Class B                  
Equity And Earnings Per Share [Line Items]                  
Shares authorized 100   100     100 100    
Common stock, shares issued 100   100     100 100    
Common stock, shares outstanding 100   100     100 100