Components of Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Postemployment Benefits [Abstract]
Service cost	$ 28.7	$ 23.8
Interest on projected benefit obligation	21.3	20.4
Expected return on assets	(44.8)	(39.8)
Amortization of prior service costs	0.4	0.3
Recognized actuarial loss	7.7	6.7
Net pension expense	$ 13.3	$ 11.4