Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8.2	$ 12.0	$ 24.6	$ 36.1
Interest cost	12.8	10.3	38.5	30.9
Expected return on assets	(17.9)	(19.3)	(53.6)	(58.0)
Prior service cost (credit)	1.5	1.3	4.4	3.7
Actuarial loss (gain)	0.2	1.0	0.5	3.0
Settlement charge	0.0	4.5	0.0	4.5
Net periodic benefit cost (credit)	4.8	9.8	14.4	20.2
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.1	0.1	0.4	0.2
Expected return on assets	0.0	0.0	0.0	0.0
Prior service cost (credit)	(0.1)	(0.2)	(0.2)	(0.5)
Actuarial loss (gain)	(0.1)	(0.1)	(0.5)	(0.2)
Settlement charge	0.0	0.0	0.0	0.0
Net periodic benefit cost (credit)	$ (0.1)	$ (0.2)	$ (0.3)	$ (0.5)