Other Comprehensive (Loss) Income - Schedule of Reclassification From Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	$ 1.0	$ 1.7	$ 4.1	$ (65.9)
Investment and other income-net	5.5	6.9	14.7	59.6
Net Actuarial Loss | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income-net	2.0	1.9	5.9	5.4
Net Realized Loss, Before Tax | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Selling, general and administrative expenses	0.0	2.8	0.0	2.8
Net Prior Service Credit | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income-net	(0.8)	(0.7)	(2.2)	(2.2)
Settlements | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income-net	0.2		1.5
Translation Adjustments
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	0.0	2.8	0.0	2.8
Amortization of Pension and OPEB Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	1.4	1.2	5.2	3.2
Income tax expense benefit	0.4	0.5	1.1	1.1
Reclassification, net of tax	1.0	0.7	4.1	2.1
Available-for-Sale Securities, Net Realized Gain on Equity Securities, Before Tax
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income tax expense benefit	0.0		0.0	18.0
Reclassification, net of tax	0.0	(1.8)	0.0	(70.8)
Available-for-Sale Securities, Net Realized Gain on Equity Securities, Before Tax | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income-net	$ 0.0	$ (1.8)	$ 0.0	$ (52.8)