Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (245.4)	$ (202.9)
Amounts reclassified from accumulated other comprehensive loss	(4.7)	1.2
Other comprehensive income	1.6	21.5
Ending Balance	(250.0)	(188.3)
Pension and OPEB Cost
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(155.2)	(144.6)
Other comprehensive income before reclassifications		1.4
Amounts reclassified from accumulated other comprehensive loss	(0.2)	1.2
Other comprehensive income	(0.2)	2.6
Ending Balance	(155.4)	(142.0)
Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1.4	40.9
Other comprehensive income before reclassifications	5.5	18.4
Amounts reclassified from accumulated other comprehensive loss	(4.5)
Other comprehensive income	1.0	18.4
Ending Balance	2.4	59.3
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive income before reclassifications	0.6
Other comprehensive income	0.6
Ending Balance	0.6
AOCI Attributable to Parent
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(153.8)	(103.7)
Other comprehensive income before reclassifications	6.1	19.8
Amounts reclassified from accumulated other comprehensive loss	(4.7)	1.2
Other comprehensive income	1.4	21.0
Ending Balance	$ (152.4)	$ (82.7)