Other Comprehensive (Loss) Income - Schedule of Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (243.8)	$ (254.4)	$ (243.8)	$ (465.9)	$ (416.6)	$ (370.3)	$ (243.8)	$ (370.3)
Amounts reclassified from accumulated other comprehensive loss	1.3			2.1			14.6	4.4
Other comprehensive (loss) income	(9.2)	17.5	(6.6)	18.5	6.2	(31.2)	1.7	(6.5)
Ending Balance	(223.6)	(243.8)	(254.4)	(454.8)	(465.9)	(416.6)	(223.6)	(454.8)
Changes in Fair Value of Derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			(11.0)			1.0	(11.0)	1.0
Other comprehensive loss before reclassifications							(2.3)	(15.1)
Amounts reclassified from accumulated other comprehensive loss	0.3			1.3			11.5	2.1
Other comprehensive (loss) income	0.2			0.3			9.2	(13.0)
Ending Balance	(1.8)			(12.0)			(1.8)	(12.0)
Pension and OPEB Cost
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			(178.5)			(185.7)	(178.5)	(185.7)
Amounts reclassified from accumulated other comprehensive loss							3.1	2.3
Other comprehensive (loss) income	1.0			0.8			3.1	2.3
Ending Balance	(175.4)			(183.4)			(175.4)	(183.4)
Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			35.6			8.5	35.6	8.5
Other comprehensive loss before reclassifications							(10.5)	4.0
Other comprehensive (loss) income							(10.5)	4.0
Ending Balance	25.1			12.5			25.1	12.5
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(142.9)	(160.3)	(153.9)	(201.0)	(207.2)	(176.2)	(153.9)	(176.2)
Other comprehensive loss before reclassifications							(12.8)	(11.1)
Amounts reclassified from accumulated other comprehensive loss							14.6	4.4
Other comprehensive (loss) income	(9.2)	17.4	(6.4)	18.1	6.2	(31.0)	1.8	(6.7)
Ending Balance	$ (152.1)	$ (142.9)	$ (160.3)	$ (182.9)	$ (201.0)	$ (207.2)	$ (152.1)	$ (182.9)