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Employee Incentive Plans, Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of common shares available for additional awards under Equity Incentive Plan 737,954    
Recognized tax benefit relating to non-cash compensation expense $ 4.9 $ 4.4 $ 3.5
Fully-vested options outstanding 0 1,543  
Exercise price of options to purchase common stock   $ 80.01  
Stock options granted 0    
Shares Paid for Tax Withholding for Share Based Compensation 56,495 36,055 37,009
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 1 year    
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Restricted stock units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of common share received by the employee on vesting of restricted stock unit 1    
TSR-Based Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
TSR-Based Performance Shares | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 0.00%    
TSR-Based Performance Shares | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 200.00%