Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension income
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.3	$ 0.3
Interest cost	5.1	6.9
Expected return on plan assets	(9.5)	(10.2)
Amortization, net	2.9	2.5
Net pension income	(1.2)	(0.5)
OPEB income
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.1	1.8
Expected return on plan assets	(3.0)	(3.2)
Amortization, net	(1.5)	(1.5)
Net pension income	$ (3.4)	$ (2.9)