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Employee Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended 140 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2008
Dec. 31, 2018
May 03, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
401(k) plan contribution expense $ 4.0 $ 3.0 $ 3.0      
Common Stock Shares issued under ROI Unit Stock Plan           1,271,009
ROI Shares issued to IBG LLC           1,250,000
2007 SIP shares issued under ROI Unit Stock Plan           21,009
Fair value ROI Treasury Stock           $ 38.0
Maximum shares of stock distributable under 2007 Stock Incentive Plan 30,000,000       30,000,000  
Shares granted to external directors         25,636  
Stock Incentive Plan Granted Shares Adjustment   23,082   23,082    
2007 Stock Incentive Plan Compensation Expense $ 58.0 $ 53.0 $ 51.0      
Estimated Future 2007 Stock Incentive Plan Compensation Expense $ 40.0       $ 40.0  
Post employment shares distribution         826,715  
2007 Stock Incentive Plan (Shares)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting Percentage, description • 10% on the first vesting date, which is on or about May 9 of each year; and• an additional 15% on each of the following six anniversaries of the first vesting, assuming continued employment with the Company and compliance with non-competition and other applicable covenants.          
2007 Stock Incentive Plan (Shares) | External Director [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percentage per year 20.00%          
Vesting period 5 years