Summary of Option Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Options	16.0	17.2	17.0
Granted, Options	2.1	2.1	2.2
Exercised, Options	(1.8)	(3.0)	(1.8)
Cancelled, Options	(0.2)	(0.3)	(0.2)
Ending Balance, Options	16.1	16.0	17.2
Exercisable as of December 31, 2017, Options	10.0
Beginning Balance, Weighted-Average Exercise Price	$ 30.06	$ 25.89	$ 22.75
Granted, Weighted-Average Exercise Price	48.39	46.75	41.91
Exercised, Weighted-Average Exercise Price	23.06	16.96	15.56
Cancelled, Weighted-Average Exercise Price	43.64	39.60	30.95
Ending Balance, Weighted-Average Exercise Price	33.11	$ 30.06	$ 25.89
Exercisable as of December 31, 2017, Weighted-Average Exercise Price	$ 25.35
Outstanding as of December 31, 2017, Weighted-Average Remaining Contractual Term, years	5 years 4 months 24 days
Exercisable as of December 31, 2017, Weighted-Average Remaining Contractual Term, years	3 years 10 months 24 days
Outstanding as of December 31, 2017, Aggregate Intrinsic Value	$ 281.6
Exercisable as of December 31, 2017, Aggregate Intrinsic Value	$ 247.5