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Share-Based Compensation - Share-Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]          
Share-based compensation $ 11.7 $ 10.3 $ 33.5 $ 30.0  
Tax benefit 2.9 $ 4.0 8.4 $ 11.5  
Compensation expense related to share-based compensation $ 67.1   $ 67.1   $ 63.5
Weighted average period over which compensation expense will be recognized     2 years