Components of Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Postemployment Benefits [Abstract]
Service cost	$ 23.5	$ 22.2	$ 69.5	$ 66.7
Interest on projected benefit obligation	20.5	23.4	60.9	71.1
Expected return on assets	(40.1)	(35.2)	(119.5)	(106.9)
Amortization of prior service costs	0.3	0.3	0.9	0.9
Recognized actuarial loss	6.3	6.9	18.9	20.8
Net pension expense	$ 10.5	$ 17.6	$ 30.7	$ 52.6