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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended 36 Months Ended 72 Months Ended 122 Months Ended
Jul. 28, 2017
Jul. 10, 2017
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2010
Dec. 31, 2016
Jun. 30, 2017
May 03, 2007
Equity And Earnings Per Share [Line Items]                    
IBG Holdings ownership % of IBG LLC     82.90%   82.90%       82.90% 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     $ 259   $ 259     $ 273 $ 259  
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     472   $ 472       472  
Tax savings owed to IBG Holdings LLC     401   401       401  
Tax savings retained by IBG Inc.     $ 71   $ 71       71  
Tax savings paid to IBG Holdings LLC   $ 15             $ 116  
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               $ 362    
Shares Issued 1,214,860             12,643,495    
Thomas Peterffy and Affiliates Ownership     88.80%   88.80%       88.80% 84.60%
Distribution from IBG LLC   88     $ 99          
Cash distribution to IBG, Inc.   $ 15     $ 17          
Dividend per share         $ 0.10          
Dividends paid to common shareholders     $ 7 $ 7 $ 14 $ 13        
Declaration Date         Jul. 18, 2017          
Payment Date         Sep. 14, 2017          
Record Date         Sep. 01, 2017          
IBG LLC [Member]                    
Equity And Earnings Per Share [Line Items]                    
IBG Inc. ownership % of IBG LLC 17.40%   17.10%   17.10%       17.10% 10.00%
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     70,384,414   70,384,414     68,119,412 70,384,414  
Common stock, shares outstanding     70,251,034   70,251,034     67,984,973 70,251,034  
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