Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension income
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.2	$ 0.3	$ 0.5	$ 0.5
Interest cost	6.8	8.3	13.7	16.7
Expected return on plan assets	(10.0)	(11.6)	(20.2)	(23.2)
Amortization, net	2.5	1.5	5.0	3.0
Net pension income	(0.5)	(1.5)	(1.0)	(3.0)
OPEB income
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1.8	2.4	3.6	5.0
Expected return on plan assets	(3.1)	(3.3)	(6.3)	(6.6)
Amortization, net	(1.5)	(1.7)	(3.0)	(3.5)
Net pension income	$ (2.8)	$ (2.6)	$ (5.7)	$ (5.1)