Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension income
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.3	$ 0.2
Interest cost	6.9	8.4
Expected return on plan assets	(10.2)	(11.6)
Amortization, net	2.5	1.5
Net pension income	(0.5)	(1.5)
OPEB income
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.8	2.6
Expected return on plan assets	(3.2)	(3.3)
Amortization, net	(1.5)	(1.8)
Net pension income	$ (2.9)	$ (2.5)