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Stock Compensation Plans (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares outstanding other than RSUs (in shares) 0 0 0
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) $ 70.13 $ 56.59 $ 29.18
Total fair value of awards that vested $ 16.2 $ 13.0 $ 4.4
Total unrecognized stock compensation expense $ 11.6    
Weighted average period to recognize unrecognized compensation cost 1 year 9 months 18 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) $ 66.86 $ 55.28 $ 31.86
Total fair value of awards that vested $ 23.1 $ 17.2 $ 4.2
Total unrecognized stock compensation expense $ 31.9    
Weighted average period to recognize unrecognized compensation cost 1 year 10 months 28 days    
2016 Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock, shares authorized (in shares) 14,000,000    
Shares available (in shares) 4,200,000