Pension and Other Postretirement Employee Benefit Plans - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	$ 0.4	$ 0.5	$ 1.4	$ 1.7
Interest cost	2.1	2.6	6.3	7.8
Expected return on plan assets	(2.6)	(3.8)	(7.9)	(11.3)
Defined Benefit Plan, Amortization of Gain (Loss)	2.5	2.4	7.7	7.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	2.4	1.7	7.4	5.6
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.2	0.0	0.3	0.1
Interest cost	0.6	0.6	1.5	1.8
Defined Benefit Plan, Amortization of Gain (Loss)	0.2	0.0	0.2	0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	1.0	0.7	2.0	1.8
Cost of Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	0.6		1.5
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		0.3		1.1
Selling, General and Administrative Expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Service Cost	$ 0.1		$ 0.2
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax		$ 0.2		$ 0.7