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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 36 Months Ended 84 Months Ended 131 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2010
Dec. 31, 2017
Mar. 31, 2018
May 03, 2007
Equity And Earnings Per Share [Line Items]            
IBG Holdings ownership % of IBG LLC 82.60%       82.60% 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  
Preferred stock shares issued 0     0 0  
Preferred stock shares outstanding 0     0 0  
Amortization period DTA (years) 15 years          
Unamortized deferred tax asset arising from equity offerings $ 142     $ 146 $ 142  
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  
Tax savings owed to IBG Holdings LLC 410       410  
Tax savings retained by IBG Inc. $ 73       73  
Tax savings paid to IBG Holdings LLC         $ 131  
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% 84.60%
Distribution from IBG LLC $ 74          
Cash distribution to IBG, Inc. $ 13          
Dividend per share $ 0.10          
Dividends paid to common shareholders $ 7 $ 7        
Declaration Date Apr. 17, 2018          
Payment Date Jun. 14, 2018          
Record Date Jun. 01, 2018          
IBG LLC [Member]            
Equity And Earnings Per Share [Line Items]            
IBG Inc. ownership % of IBG LLC 17.40%       17.40% 10.00%
Common Class A            
Equity And Earnings Per Share [Line Items]            
Common stock, par value $ 0.01     $ 0.01 $ 0.01  
Shares authorized 1,000,000,000     1,000,000,000 1,000,000,000  
Common stock, shares issued 71,610,581     71,609,049 71,610,581  
Common stock, shares outstanding 71,481,133     71,475,755 71,481,133  
Common Class B            
Equity And Earnings Per Share [Line Items]            
Shares authorized 100     100 100  
Common stock, shares issued 100     100 100  
Common stock, shares outstanding 100     100 100