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Equity And Earnings Per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 9 Months Ended 36 Months Ended 60 Months Ended 113 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2010
Dec. 31, 2015
Sep. 30, 2016
Jun. 30, 2016
May 03, 2007
Equity And Earnings Per Share [Line Items]              
IBG Inc. ownership % of IBG LLC 16.60%       16.60% 16.20% 10.00%
IBG Holdings ownership % of IBG LLC 83.40%       83.40%   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 $ 279     $ 288 $ 279    
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    
Tax savings owed to IBG Holdings LLC 401       401    
Tax savings retained by IBG Inc. 71       71    
Tax savings paid to IBG Holdings LLC         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 For Membership Exchange $ 306       $ 306    
Shares Issued 1,596,200     11,047,295      
Thomas Peterffy and Affiliates Ownership 89.10%       89.10%   84.60%
Distribution from IBG LLC $ 226            
Cash distribution to IBG, Inc. $ 37            
Dividend per share $ 0.10            
Dividends paid to common shareholders $ 20 $ 18          
Declaration Date Oct. 18, 2016            
Payment Date Dec. 14, 2016            
Record Date Dec. 01, 2016            
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 68,117,402     64,121,150 68,117,402    
Common stock, shares outstanding 67,982,963     63,985,335 67,982,963    
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