Components of Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Postemployment Benefits [Abstract]
Service cost	$ 26.0	$ 22.2	$ 54.7	$ 46.0
Interest on projected benefit obligation	21.7	20.0	43.0	40.4
Expected return on assets	(44.5)	(39.6)	(89.3)	(79.4)
Amortization of prior service costs	0.3	0.3	0.7	0.6
Recognized actuarial loss	10.1	5.9	17.8	12.6
Net pension expense	$ 13.6	$ 8.8	$ 26.9	$ 20.2