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Stock-Based Compensation Plans (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Jan. 01, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Common stock available under stock option plan (in shares) 11,273,436   11,273,436      
Vesting period     3 years      
Options, outstanding, number (in shares) 114,943   114,943      
Options, grants in period, gross (in shares)     0 0    
Plan 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares authorized to issue under share based compensation plans (in shares)           12,500,000
Restricted Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based compensation arrangement age requirement to vest over original vesting period     55 years      
Time in service requirement to vest over original vesting period     10 years      
Nonvested awards, compensation cost not yet recognized $ 38.1   $ 38.1   $ 18.7  
Nonvested awards, compensation cost not yet recognized, period for recognition     2 years      
Equity instruments other than options, vested in period, fair value 0.9 $ 2.9 $ 24.4 $ 21.0    
Allocated share-based compensation expense, net of tax 5.1 3.3 12.7 14.4    
Allocated share-based compensation expense $ 6.7 $ 4.2 $ 16.5 $ 18.5    
Equity instruments other than options, nonvested, number (in shares) 1,701,401   1,701,401   1,373,657  
Performance Shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Nonvested awards, compensation cost not yet recognized, period for recognition     36 months      
Equity instruments other than options, nonvested, number (in shares) 509,000   509,000      
Total shareholder return performance measure, as a percentage 15.00%   15.00%      
Estimated vesting of shares based on performance shares (in shares) 480,000   480,000      
Minimum | Performance Shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options, vested and expected to vest, exercisable, number (in shares) 0   0      
Minimum | Performance Shares | 2019 and 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     0.00%      
Maximum | Performance Shares            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Options, vested and expected to vest, exercisable, number (in shares) 1,063,000   1,063,000      
Maximum | Performance Shares | 2019 and 2020            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     200.00%      
Maximum | Performance Shares | 2021            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Award vesting rights, percentage     230.00%