Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (465.9)	$ (416.6)	$ (370.3)	$ (270.8)	$ (250.0)	$ (245.4)	$ (370.3)	$ (245.4)
Amounts reclassified from accumulated other comprehensive loss	2.1			(0.2)			4.4	(5.1)
Other comprehensive income (loss)	18.5	6.2	(31.2)	(19.5)	(2.5)	1.6	(6.5)	(20.4)
Ending Balance	(454.8)	(465.9)	(416.6)	(276.9)	(270.8)	(250.0)	(454.8)	(276.9)
Changes in Fair Value of Derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			1.0				1.0
Other comprehensive loss before reclassifications							(15.1)
Amounts reclassified from accumulated other comprehensive loss	1.3						2.1
Other comprehensive income (loss)	0.3						(13.0)
Ending Balance	(12.0)						(12.0)
Pension and OPEB Cost
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			(185.7)			(155.2)	(185.7)	(155.2)
Amounts reclassified from accumulated other comprehensive loss							2.3	(0.6)
Other comprehensive income (loss)	0.8			(0.2)			2.3	(0.6)
Ending Balance	(183.4)			(155.8)			(183.4)	(155.8)
Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			8.5			1.4	8.5	1.4
Other comprehensive loss before reclassifications							4.0	(17.1)
Amounts reclassified from accumulated other comprehensive loss								(4.5)
Other comprehensive income (loss)							4.0	(21.6)
Ending Balance	12.5			(20.2)			12.5	(20.2)
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive loss before reclassifications								2.1
Other comprehensive income (loss)				0.6				2.1
Ending Balance				2.1				2.1
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(201.0)	(207.2)	(176.2)	(154.8)	(152.4)	(153.8)	(176.2)	(153.8)
Other comprehensive loss before reclassifications							(11.1)	(15.0)
Amounts reclassified from accumulated other comprehensive loss							4.4	(5.1)
Other comprehensive income (loss)	18.1	6.2	(31.0)	(19.1)	(2.4)	1.4	(6.7)	(20.1)
Ending Balance	$ (182.9)	$ (201.0)	$ (207.2)	$ (173.9)	$ (154.8)	$ (152.4)	$ (182.9)	$ (173.9)