Share Plans - Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Shares
Beginning balance	5.7
Granted	0.9
Exercised	(1.2)	(0.1)
Forfeited	(0.2)
Expired	0.0
Ending balance	5.2
Options exercisable	3.4
Options expected to vest at end of period	1.8
Weighted Average Exercise Price
Beginning balance (USD per share)	$ 45.72
Granted (USD per share)	59.09
Exercised (USD per share)	37.00
Forfeited (USD per share)	60.03
Expired (USD per share)	0.00
Ending Balance (USD per share)	49.49
Options exercisable at end of period (USD per share)	46.00
Options expected to vest at end of period (USD per share)	$ 56.13
Weighted Average Remaining Contractual Life
Ending balance	5 years 5 months 30 days
Options exercisable at end of period	4 years
Options expected to vest at end of period	8 years 3 months 30 days
Aggregate Intrinsic value
Balance at end of period	$ 113.7
Options exercisable at end of period	87.5
Options expected to vest at end of period	$ 25.8