Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension income
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.3	$ 0.2	$ 0.9	$ 0.7
Interest cost	5.1	6.8	15.3	20.5
Expected return on plan assets	(9.6)	(10.2)	(28.7)	(30.4)
Amortization, net	2.9	2.5	8.8	7.5
Net pension income	(1.3)	(0.7)	(3.7)	(1.7)
OPEB income
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.1	1.9	3.2	5.5
Expected return on plan assets	(3.0)	(3.1)	(8.9)	(9.4)
Amortization, net	(1.5)	(1.5)	(4.7)	(4.5)
Net pension income	$ (3.4)	$ (2.7)	$ (10.4)	$ (8.4)