Annual report pursuant to Section 13 and 15(d)

Share Based Compensation (Narrative) (Details)

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Share Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Additional compensation expense $ 5.2 $ 4.0 $ 32.5
Share-based compensation arrangement by share-based payment award, accelerated compensation cost     $ 16.6
Unrecognized compensation costs 12.8    
Compensation costs expected to be recognized in 2020 4.5    
Compensation costs expected to be recognized in 2021 $ 3.5    
Weighted average values of grants (in dollars per share) $ 27.67 $ 29.72 $ 37.20
Aggregate intrinsic value of exercised stock options $ 2.5 $ 3.9 $ 5.6
Weighted average remaining contractual term for total outstanding stock options 5 years 1 month 6 days    
Weighted average remaining contractual term for vested/exercisable stock options 3 years 10 months 24 days    
Aggregate intrinsic value of options outstanding $ 3.8    
Aggregate intrinsic value of vested/exercisable options outstanding 2.7    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total grant date fair value of restricted stock vested $ 1.2   $ 26.1
Employee Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock authorized for grant (in shares) 900,000    
Exercise price per share of options granted, percentage, minimum 85.00%    
Increase in number of shares available under employee stock purchase plan (in shares)   300,000  
Employee common stock purchase plan, duration   10 years  
Employee common stock purchase plan, offering period six    
Employee Stock Options and Stock Options Issued for Services      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average expected volatility 28.20% 26.60% 25.50%
Weighted average discount rates 1.88% 2.78% 1.92%
Expected lives 5 years 9 months 5 years 6 months 29 days 5 years 7 months 2 days
Share Incentive Plans      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock authorized for grant (in shares) 6,650,000    
Exercise price per share of options granted, percentage, minimum 100.00%    
Share Incentive Plans | Restricted Stock | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock and stock options vesting period 1 year    
Share Incentive Plans | Restricted Stock | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock and stock options vesting period 5 years    
Share Incentive Plans | Employee Stock Option | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock and stock options vesting period 1 year    
Share Incentive Plans | Employee Stock Option | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock and stock options vesting period 5 years