Stock Based Compensation Expense (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based Payment Award
Stock-based compensation expense		$ 84.5	$ 88.0	$ 72.6
Tax benefit		30.4	32.1	26.3
Employee Stock Option
Stock-based Payment Award
Stock-based compensation expense		0.0	0.2	1.0
Stock-settled awards
Stock-based Payment Award
Stock-based compensation expense		20.5	16.0	11.6
Cash-settled awards
Stock-based Payment Award
Stock-based compensation expense		1.0	0.9	0.7
Performance-based awards
Stock-based Payment Award
Stock-based compensation expense		61.5	69.4	57.7
Employee stock purchase plan
Stock-based Payment Award
Stock-based compensation expense		1.0	0.7	0.7
Other
Stock-based Payment Award
Stock-based compensation expense	[1]	$ 0.5	$ 0.8	$ 0.9

[1] Represents charges recorded for severance expense related to stock-based compensation awards.