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Share-Based Compensation - Share-Based Compensation Expense (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]          
Share-based compensation $ 10.2 $ 11.1 $ 19.0 $ 21.8  
Tax benefit 2.5 $ 2.8 4.7 $ 5.5  
Compensation expense related to share-based compensation $ 70.8   $ 70.8   $ 54.1
Weighted average period over which compensation expense will be recognized     2 years