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Employee Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended 128 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
May 03, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
401(k) plan contribution expense $ 3.0 $ 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
Common stock to be distributed to former employees under the ROI Unit Stock Plan 3,849        
Maximum shares of stock distributable under 2007 Stock Incentive Plan 30,000,000     30,000,000  
Shares granted to external directors       24,263  
Stock Incentive Plan Granted Shares Adjustment   5,657      
2007 Stock Incentive Plan Compensation Expense $ 53.0 $ 51.0 $ 50.0    
Estimated Future 2007 Stock Incentive Plan Compensation Expense $ 38.0     $ 38.0  
Post employment shares distribution       573,197  
2007 Stock Incentive Plan (Shares)          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting Percentage • 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.