Summary of Option Activity (Detail) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Shares
Shares, Outstanding, Beginning Balance | shares	3.9
Shares, Granted | shares	0.2
Shares, Exercised | shares	(1.2)
Shares, Outstanding, Ending Balance | shares	2.9
Shares, Vested and expected to vest | shares	2.8
Shares, Exercisable | shares	2.1
Weighted Average Exercise Price
Weighted Average Exercise Price Per Share, Beginning Balance | $ / shares	$ 49.68
Weighted Average Exercise Price Per Share, Granted | $ / shares	113.8
Weighted Average Exercise Price Per Share, Exercised | $ / shares	42.52
Weighted Average Exercise Price Per Share, Ending Balance | $ / shares	57.48
Weighted Average Exercise Price Per Share, Vested and expected to vest | $ / shares	56.5
Weighted Average Exercise Price Per Share, Exercisable | $ / shares	$ 43.3
Weighted Average Remaining Contractual Term
Weighted Average Remaining Contractual Term, Outstanding	4 years 8 months 12 days
Weighted Average Remaining Contractual Term, Vested and expected to vest	4 years 7 months 6 days
Weighted Average Remaining Contractual Term	3 years 4 months 24 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding | $	$ 261.6
Aggregate Intrinsic Value, Vested and expected to vest | $	257.7
Aggregate Intrinsic Value, Exercisable | $	$ 216.8