Significant Accounting Policies - Reclassifications Out of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Additional interest expense	$ 28.7	$ 32.4	$ 89.7	$ 98.7
Tax benefit	81.5	66.2	143.0	111.1
Pension and Postretirement Benefit Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Earnings from Continuing Operations	3.1	2.7	8.0	8.2
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefit Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service credit		(0.2)	(0.1)	(0.6)
Actuarial loss	4.2	3.8	10.8	11.5
Tax benefit	(1.1)	(0.9)	(2.7)	(2.7)
Earnings from Continuing Operations	3.1	2.7	8.0	8.2
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefit Plans | Other Nonoperating (Income) and Expense, Net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications out of accumulated other comprehensive loss before taxes	$ 4.2	$ 3.6	$ 10.7	$ 10.9