Summary of Option Activity (Details) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Options | shares	16.1
Granted, Options | shares	1.9
Exercised, Options | shares	(1.5)
Cancelled, Options | shares	(0.1)
Ending Balance, Options | shares	16.4
Exercisable at June 30, 2018, Options | shares	10.5
Beginning Balance, Weighted-Average Exercise Price | $ / shares	$ 33.11
Granted, Weighted-Average Exercise Price | $ / shares	50.22
Exercised, Weighted-Average Exercise Price | $ / shares	19.28
Cancelled, Weighted-Average Exercise Price | $ / shares	47.73
Ending Balance, Weighted-Average Exercise Price | $ / shares	36.22
Exercisable at June 30, 2018, Weighted-Average Exercise Price | $ / shares	$ 29.19
Outstanding at June 30, 2018, Weighted-Average Remaining Contractual Term, years	5 years 9 months 18 days
Exercisable at June 30, 2018, Weighted-Average Remaining Contractual Term, years	4 years 3 months 18 days
Outstanding at June 30, 2018, Aggregate Intrinsic Value | $	$ 279.5
Exercisable at June 30, 2018, Aggregate Intrinsic Value | $	$ 251.8