Components of Net Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 24.7	$ 28.7
Interest on projected benefit obligation	23.7	21.3
Expected return on assets	(44.6)	(44.8)
Amortization of prior service costs	0.4	0.4
Recognized actuarial loss	4.4	7.7
Net pension expense	$ 8.6	$ 13.3