Other Comprehensive Loss - Schedule of Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (250.0)	$ (245.4)	$ (188.3)	$ (202.9)	$ (245.4)	$ (202.9)
Amounts reclassified from accumulated other comprehensive loss	(0.2)		1.9		(4.9)	3.1
Other comprehensive loss	(2.5)	1.6	(39.9)	21.5	(0.9)	(18.4)
Ending Balance	(270.8)	(250.0)	(247.5)	(188.3)	(270.8)	(247.5)
Pension and OPEB Cost
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(155.2)		(144.6)	(155.2)	(144.6)
Other comprehensive income (loss) before reclassifications						1.2
Amounts reclassified from accumulated other comprehensive loss					(0.4)	3.1
Other comprehensive loss	(0.2)		1.7		(0.4)	4.3
Ending Balance	(155.6)		(140.3)		(155.6)	(140.3)
Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		1.4		40.9	1.4	40.9
Other comprehensive income (loss) before reclassifications					2.4	(22.6)
Amounts reclassified from accumulated other comprehensive loss					(4.5)
Other comprehensive loss					(2.1)	(22.6)
Ending Balance	(0.7)		18.3		(0.7)	18.3
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive income (loss) before reclassifications					1.5
Other comprehensive loss	0.9				1.5
Ending Balance	1.5				1.5
AOCI Attributable to Parent
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		$ (153.8)		$ (103.7)	(153.8)	(103.7)
Other comprehensive income (loss) before reclassifications					3.9	(21.4)
Amounts reclassified from accumulated other comprehensive loss					(4.9)	3.1
Other comprehensive loss					(1.0)	(18.3)
Ending Balance	$ (154.8)		$ (122.0)		$ (154.8)	$ (122.0)