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Stock Compensation Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
RSUs | Ratably      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
RSUs | Cliff | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
RSUs | Cliff | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 5 years    
Performance-Based RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of awards granted (in dollars per share) $ 39.79 $ 54.64 $ 70.13
Total fair value of awards that vested $ 8.9 $ 14.5 $ 16.2
Total unrecognized stock compensation expense $ 13.0    
Weighted average period to recognize unrecognized compensation cost 1 year 8 months 19 days    
Performance-Based RSUs | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of target award available for grant 0.00%    
Performance-Based RSUs | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of target award available for grant 200.00%    
Performance-Based RSUs | Cliff      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Time-Based RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average fair value of awards granted (in dollars per share) $ 40.61 $ 52.46 $ 66.86
Total fair value of awards that vested $ 11.5 $ 20.7 $ 23.1
Total unrecognized stock compensation expense $ 22.8    
Weighted average period to recognize unrecognized compensation cost 1 year 8 months 19 days    
Awards Other Than RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares outstanding other than RSUs (in shares) 0 0 0
2016 Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock, shares authorized (in shares) 14,000,000.0    
Shares available (in shares) 2,800,000