Other Comprehensive (Loss) Income - Schedule of Reclassification From Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	$ 1.9	$ (96.9)	$ 3.1	$ (67.6)
Investment and other income-net	3.6	97.3	9.2	52.7
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	1.9	1.8	3.9	3.5
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	(0.7)	(0.8)	(1.4)	(1.5)
Settlements | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income-net	1.0		1.3
Amortization of Pension and Other Postretirement Benefits Plan Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	2.2	1.0	3.8	2.0
Income tax expense benefit	0.3	0.3	0.7	0.6
Reclassification, net of tax	$ 1.9	0.7	$ 3.1	1.4
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				18.0
Reclassification, net of tax		(97.6)		(69.0)
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		$ (97.6)		$ (51.0)