Components of Net Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 32.8	$ 25.3	$ 98.0	$ 77.1
Interest on projected benefit obligation	20.8	23.7	62.1	71.7
Expected return on assets	(47.6)	(43.8)	(142.3)	(132.3)
Amortization of prior service costs	0.4	0.4	1.1	1.1
Recognized actuarial loss	11.1	5.0	33.1	15.3
Settlement loss	0.5		8.5
Net pension expense	$ 18.0	$ 10.6	$ 60.5	$ 32.9