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Share-Based Compensation Plans - Restricted Stock Narratives (Details) - USD ($)
$ 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
Restricted stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Term for restrictions to lapse 6 years    
Share-based compensation $ 12.3 $ 13.8 $ 9.9
Total unrecognized compensation cost, net of estimated forfeitures $ 14.3    
Total unrecognized compensation cost, weighted-average period for recognition 1 year 10 months 24 days