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Stock-Based Compensation Plans (Information Regarding Restricted Shares) (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
General vesting period (years) 3 years    
Unearned compensation costs $ 18.5 $ 18.0  
Recognition of unearned compensation, (years) 1 year    
Fair value of Restricted Shares vested $ 24.2 23.5 $ 25.2
Stock-based compensation expense 27.8 25.5 29.5
Related income tax benefit (6.3) (5.8) (6.7)
Net stock-based compensation expense $ 21.5 $ 19.7 $ 22.8
Shares      
Outstanding — beginning of year (in shares) 1,697,779    
Granted (in shares) 936,683    
Vested (in shares) (645,946)    
Canceled (in shares) (247,030)    
Outstanding — ending of year (in shares) 1,741,486 1,697,779  
Weighted Average Grant-Date Fair Value      
Outstanding — beginning of year (in dollars per share) $ 37.17    
Granted (in dollars per share) 36.72    
Vested (in dollars per share) 37.48    
Canceled (in dollars per share) 42.42    
Outstanding — ending of year (in dollars per share) $ 36.06 $ 37.17  
Restricted Stock | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
General expiration period, from date of grant or time of termination (years) 1 year    
General vesting period (years) 1 year    
Restricted Stock | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
General expiration period, from date of grant or time of termination (years) 3 years    
General vesting period (years) 3 years    
Performance Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
General vesting period (years) 36 months    
Shares      
Outstanding — ending of year (in shares) 473,000    
Weighted Average Grant-Date Fair Value      
Total shareholder return performance measure, as a percentage 15.00%    
Estimated vesting of shares based on performance shares 370,000    
Performance Shares | Minimum      
Weighted Average Grant-Date Fair Value      
Estimated vesting of shares based on performance shares 0    
Performance Shares | Maximum      
Weighted Average Grant-Date Fair Value      
Estimated vesting of shares based on performance shares 1,087,000    
Performance Shares | Maximum | Issued in 2021      
Weighted Average Grant-Date Fair Value      
Vesting percentage of grants, depending on achievement of specific performance targets 230.00%