Pension and Other Postretirement Employee Benefit Plans - Narrative (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Anti-dilutive shares excluded from calculation	0.4	1.1	0.6	1.0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	$ 0.5	$ 0.6	$ 1.7	$ 1.8
Interest cost	2.6	3.1	7.8	9.3
Expected return on plan assets	(3.8)	(4.1)	(11.3)	(12.4)
Amortization of actuarial loss	2.4	1.8	7.4	5.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	1.7	1.4	5.6	4.3
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.0	0.0	0.1	0.1
Interest cost	0.6	0.7	1.8	2.1
Amortization of actuarial loss	0.0	(0.1)	0.0	(0.3)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	0.7	0.6	1.8	1.9
Cost of Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax	0.3	0.4	1.1	1.1
Selling, General and Administrative Expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax	$ 0.2	$ 0.3	$ 0.7	$ 0.8