Share-Based Compensation - Allocation of Compensation Expense (Benefit) by Plan (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-Based Compensation
Share-based compensation expense (benefit) before tax	$ 23.9	$ 51.4	$ 15.9	$ (29.2)
Stock Options
Share-Based Compensation
Share-based compensation expense (benefit) before tax	18.4	16.6	52.2	44.8
Restricted Stock Units
Share-Based Compensation
Share-based compensation expense (benefit) before tax	3.8	2.4	9.8	5.3
STAP awards
Share-Based Compensation
Share-based compensation expense (benefit) before tax	1.4	32.2	(47.0)	(80.1)
Employee Stock Purchase Plan
Share-Based Compensation
Share-based compensation expense (benefit) before tax	$ 0.3	$ 0.2	$ 0.9	$ 0.8