Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11.6	$ 9.7
Interest cost	9.0	9.2
Expected return on assets	(17.6)	(14.6)
Prior service cost (credit)	0.2	0.2
Actuarial loss (gain)	3.3	3.3
Net periodic benefit cost (credit)	6.5	7.8
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.1	0.1
Prior service cost (credit)	(0.2)	(0.2)
Actuarial loss (gain)	(0.1)
Net periodic benefit cost (credit)	$ (0.2)	$ (0.1)