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Share-Based Compensation Plans - Stock Options Narratives (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 28.7 $ 27.1 $ 31.1
Stock option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based payment award vesting period 4 years    
Expiration period 10 years    
Share-based compensation $ 3.7 3.0 3.2
Fair value of options vested 2.3 $ 2.5 $ 3.2
Total unrecognized compensation cost, net of estimated forfeitures $ 2.3    
Total unrecognized compensation cost, weighted-average period for recognition 1 year 2 months 12 days    
Weighted average of daily historical volatility of Company's stock price, weight (as a percent) 75.00%    
Implied volatility based on exchange traded options for Company's common stock, weight (as a percent) 25.00%    
Weighted-average grant-date fair value of options granted (in dollars per share) $ 25.58 $ 19.68 $ 21.66
Stock option | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase price (as a percent) 100.00%