Other Comprehensive Income (Loss) - Schedule of Reclassification From Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	$ 1.2	$ 29.3
Investment and other (income) expense-net	5.6	(44.6)
Net Actuarial Loss | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other (income) expense-net	2.0	1.7
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) expense-net	(0.7)	(0.7)
Settlements | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other (income) expense-net	0.3
Amortization of Pension and Other Postretirement Benefits Plan Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	1.6	1.0
Income tax expense benefit	0.4	0.3
Reclassification, net of tax	$ 1.2	0.7
Available-for-Sale Securities, Loss on Equity Securities, Before Tax
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income tax expense benefit		18.0
Reclassification, net of tax		28.6
Available-for-Sale Securities, Loss 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) expense-net		$ 46.6