Components of Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Postemployment Benefits [Abstract]
Service cost	$ 27.0	$ 23.5	$ 81.7	$ 69.5
Interest on projected benefit obligation	21.3	20.5	64.3	60.9
Expected return on assets	(43.9)	(40.1)	(133.2)	(119.5)
Amortization of prior service costs	0.3	0.3	1.0	0.9
Recognized actuarial loss	8.7	6.3	26.5	18.9
Net pension expense	$ 13.4	$ 10.5	$ 40.3	$ 30.7