Components of Net Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 36.0	$ 32.9
Interest on projected benefit obligation	16.6	20.6
Expected return on assets	(51.1)	(47.4)
Amortization of prior service costs	0.2	0.4
Recognized actuarial loss	14.2	10.3
Settlement loss		7.0
Net pension expense	$ 15.9	$ 23.8