Pension and Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 10.4	$ 7.6	$ 19.6	$ 15.4
Interest cost	9.8	9.3	18.5	18.9
Expected return on assets	(14.8)	(11.8)	(28.6)	(23.9)
Prior service cost (credit)	0.2		0.3
Actuarial loss (gain)	3.2	3.9	6.7	7.9
Net periodic benefit cost (credit)	8.8	9.0	16.5	18.3
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.1	0.1	0.2	0.3
Prior service cost (credit)	(0.2)	(0.2)	(0.4)	(0.4)
Actuarial loss (gain)		(0.1)	(0.1)	(0.2)
Net periodic benefit cost (credit)	$ (0.1)	$ (0.2)	$ (0.3)	$ (0.3)