Significant Accounting Policies - Reclassifications Out of 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]
Tax benefit	$ 63.6	$ 81.5	$ 141.7	$ 143.0
Pension and Postretirement Benefit Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Earnings from Continuing Operations	2.2	3.1	6.8	8.0
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				(0.1)
Total reclassifications out of accumulated other comprehensive loss before taxes	3.0	4.2	9.1	10.7
Tax benefit	(0.8)	(1.1)	(2.3)	(2.7)
Earnings from Continuing Operations	2.2	3.1	6.8	8.0
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefit Plans | Other Nonoperating Income, Net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Actuarial loss	$ 3.0	$ 4.2	$ 9.1	$ 10.8