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Share-Based Compensation - Share-Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]          
Share-based compensation $ 11.1 $ 10.3 $ 21.8 $ 19.7  
Tax benefit 2.8 $ 4.0 5.5 $ 7.6  
Compensation expense related to share-based compensation $ 77.7   $ 77.7   $ 63.5
Weighted average period over which compensation expense will be recognized     2 years