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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 9 Months Ended 36 Months Ended 84 Months Ended 137 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2010
Dec. 31, 2017
Sep. 30, 2018
Jul. 27, 2018
Jul. 26, 2018
May 03, 2007
Equity And Earnings Per Share [Line Items]                      
IBG Holdings ownership % of IBG LLC 81.90%     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 $ 145     $ 145     $ 146 $ 145      
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 501     $ 501       501      
Tax savings owed to IBG Holdings LLC 426     426       426      
Tax savings retained by IBG Inc. 75     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     $ 94     $ 410 $ 94      
Shares Issued             13,858,355        
Thomas Peterffy and Affiliates Ownership 89.60%     89.60%       89.60%     84.60%
Distribution from IBG LLC       $ 376              
Cash distribution to IBG, Inc.       $ 66              
Dividend per share       $ 0.10              
Dividends paid to common shareholders $ 8 $ 7 $ 7 $ 22 $ 21            
Payment Date       Dec. 14, 2018              
Record Date       Nov. 30, 2018              
IBG LLC [Member]                      
Equity And Earnings Per Share [Line Items]                      
IBG Inc. ownership % of IBG LLC 18.10%     18.10%       18.10%   17.80% 10.00%
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 75,226,888     75,226,888     71,609,049 75,226,888      
Common stock, shares outstanding 75,097,440     75,097,440     71,475,755 75,097,440      
Common Class B                      
Equity And Earnings Per Share [Line Items]                      
Common stock, par value $ 0.01     $ 0.01     $ 0.01 $ 0.01      
Shares authorized 100     100     100 100      
Common stock, shares issued 100     100     100 100      
Common stock, shares outstanding 100     100     100 100