Summary of Option Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Options	17.2	17.0	17.5
Granted, Options	2.1	2.2	2.3
Exercised, Options	(3.0)	(1.8)	(2.6)
Cancelled, Options	(0.3)	(0.2)	(0.2)
Ending Balance, Options	16.0	17.2	17.0
Exercisable as of December 31, 2016, Options	9.7
Beginning Balance, Weighted-Average Exercise Price	$ 25.89	$ 22.75	$ 19.74
Granted, Weighted-Average Exercise Price	46.75	41.91	34.81
Exercised, Weighted-Average Exercise Price	16.96	15.56	12.57
Cancelled, Weighted-Average Exercise Price	39.60	30.95	28.16
Ending Balance, Weighted-Average Exercise Price	30.06	$ 25.89	$ 22.75
Exercisable as of December 31, 2016, Weighted-Average Exercise Price	$ 22.91
Outstanding as of December 31, 2016, Weighted-Average Remaining Contractual Term, years	5 years 9 months 18 days
Exercisable as of December 31, 2016, Weighted-Average Remaining Contractual Term, years	4 years 3 months 18 days
Outstanding as of December 31, 2016, Aggregate Intrinsic Value	$ 233.5
Exercisable as of December 31, 2016, Aggregate Intrinsic Value	$ 207.0