Summary of Option Activity (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Options | shares	16.1
Exercised, Options | shares	(0.9)
Cancelled, Options | shares	(0.1)
Ending Balance, Options | shares	15.1
Exercisable at March 31, 2018, Options | shares	9.0
Beginning Balance, Weighted-Average Exercise Price | $ / shares	$ 33.11
Exercised, Weighted-Average Exercise Price | $ / shares	17.73
Cancelled, Weighted-Average Exercise Price | $ / shares	48.20
Ending Balance, Weighted-Average Exercise Price | $ / shares	33.99
Exercisable at March 31, 2018, Weighted-Average Exercise Price | $ / shares	$ 26.13
Outstanding at March 31, 2018, Weighted-Average Remaining Contractual Term, years	5 years 4 months 24 days
Exercisable at March 31, 2018, Weighted-Average Remaining Contractual Term, years	3 years 10 months 24 days
Outstanding at March 31, 2018, Aggregate Intrinsic Value | $	$ 253.8
Exercisable at March 31, 2018, Aggregate Intrinsic Value | $	$ 219.1