Other Comprehensive (Loss) Income - Schedule of Reclassification from Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	$ 1.3	$ 2.1	$ 14.6	$ 4.4
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	2.7	3.9	8.1	6.9
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	(1.3)	(2.9)	(4.0)	(3.9)
Amortization of Pension and OPEB Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	1.4	1.0	4.1	3.0
Income tax benefit	0.4	0.2	1.0	0.7
Reclassification, net of tax	1.0	0.8	3.1	2.3
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	0.3	1.3	11.5	2.1
Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest | Reclassification out of Accumulated Other Comprehensive Income | Net Realized Loss on Derivatives
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized loss	$ 0.3	$ 1.3	$ 11.5	$ 2.1