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Employee Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended 152 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2008
Dec. 31, 2019
May 03, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 50.00%          
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 10.00%          
Defined Contribution Plan Vesting Period 6 years          
401(k) plan contribution expense $ 4.0 $ 4.0 $ 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         27,245  
Stock Incentive Plan Granted Shares Adjustment   640   640    
2007 Stock Incentive Plan Compensation Expense $ 60.0 $ 58.0 $ 53.0      
Estimated Future 2007 Stock Incentive Plan Compensation Expense $ 40.0       $ 40.0  
Post employment shares distribution         977,905  
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