Components of Net Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 32.9	$ 24.7
Interest on projected benefit obligation	20.6	23.7
Expected return on assets	(47.4)	(44.6)
Amortization of prior service costs	0.4	0.4
Recognized actuarial loss	10.3	4.4
Settlement loss	7.0
Net pension expense	$ 23.8	$ 8.6