Accounting Policies - Components of Changes in Accumulated Other Comprehensive Loss and Related Cumulative Noncurrent Deferred Tax Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 5,353.3	$ 4,949.4	$ 4,682.5
Other comprehensive (loss) earnings before reclassifications, net of Tax	(26.0)	(13.3)	(25.0)
Amounts reclassified from accumulated other comprehensive loss, net of tax	13.4	11.1	10.5
Other comprehensive (loss) earnings, net of tax	(12.6)	(2.2)	(14.5)
Ending Balance	5,893.3	5,353.3	4,949.4
Cumulative noncurrent deferred tax assets at end of period	89.4	85.2	84.2
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(144.9)	(141.5)	(128.8)
Other comprehensive (loss) earnings before reclassifications, net of Tax	(26.6)	(14.5)	(22.9)
Amounts reclassified from accumulated other comprehensive loss, net of tax	13.4	11.1	10.2
Other comprehensive (loss) earnings, net of tax	(13.2)	(3.4)	(12.7)
Ending Balance	(158.1)	(144.9)	(141.5)
Cumulative noncurrent deferred tax assets at end of period	89.4	85.2	84.2
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(0.9)	(2.1)
Other comprehensive (loss) earnings before reclassifications, net of Tax	0.6	1.2	(2.1)
Other comprehensive (loss) earnings, net of tax	0.6	1.2	(2.1)
Ending Balance	(0.3)	(0.9)	(2.1)
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(0.3)
Amounts reclassified from accumulated other comprehensive loss, net of tax			0.3
Other comprehensive (loss) earnings, net of tax			0.3
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(145.8)	(143.6)	(129.1)
Other comprehensive (loss) earnings before reclassifications, net of Tax	(26.0)
Ending Balance	$ (158.4)	$ (145.8)	$ (143.6)