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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended 36 Months Ended 84 Months Ended 140 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2010
Dec. 31, 2017
Dec. 31, 2018
May 03, 2007
Equity And Earnings Per Share [Line Items]              
IBG Holdings ownership % of IBG LLC 81.90%         81.90% 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 $ 140 $ 146     $ 146 $ 140  
Percent of tax savings owed to IBG Holdings LLC 85.00%            
Percentage of tax savings retained by IBG Inc. 15.00%            
Gain on remeasurement of Tax Receivable Agreement liability $ 3 93          
Deferred tax asset from common stock offerings 501         501  
Tax savings owed to IBG Holdings LLC 426         426  
Tax savings retained by IBG Inc. 75         75  
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 $ 94 410     $ 410 $ 94  
Shares Issued 1,537,727       13,858,355    
Thomas Peterffy and Affiliates Ownership 89.60%         89.60% 84.60%
Distribution from IBG LLC $ 426 328 $ 267        
Cash distribution to IBG, Inc. $ 76 56 43        
Dividend per share $ 0.10            
Dividends paid to common shareholders $ 29 $ 28 $ 26        
Declaration Date Jan. 22, 2019            
Payment Date Mar. 14, 2019            
Record Date Mar. 01, 2019            
IBG LLC [Member]              
Equity And Earnings Per Share [Line Items]              
IBG Inc. ownership % of IBG LLC 18.10%         18.10% 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 75,230,400 71,609,049     71,609,049 75,230,400  
Common stock, shares outstanding 75,100,952 71,475,755     71,475,755 75,100,952  
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