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, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 4,949.4	$ 4,682.5	$ 4,142.6
Other comprehensive (loss) earnings before reclassifications, net of Tax	(13.3)	(25.0)	(6.9)
Amounts reclassified from accumulated other comprehensive loss, net of tax	11.1	10.5	8.5
Other comprehensive (loss) earnings, net of tax	(2.2)	(14.5)	1.6
Ending Balance	5,353.3	4,949.4	4,682.5
Cumulative noncurrent deferred tax assets at end of period	85.2	84.2	80.1
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(141.5)	(128.8)	(128.4)
Other comprehensive (loss) earnings before reclassifications, net of Tax	(14.5)	(22.9)	(8.1)
Amounts reclassified from accumulated other comprehensive loss, net of tax	11.1	10.2	7.7
Other comprehensive (loss) earnings, net of tax	(3.4)	(12.7)	(0.4)
Ending Balance	(144.9)	(141.5)	(128.8)
Cumulative noncurrent deferred tax assets at end of period	85.2	84.2	79.9
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(2.1)		(1.2)
Other comprehensive (loss) earnings before reclassifications, net of Tax	1.2	(2.1)	1.2
Other comprehensive (loss) earnings, net of tax	1.2	(2.1)	1.2
Ending Balance	(0.9)	(2.1)
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(0.3)	(1.1)
Amounts reclassified from accumulated other comprehensive loss, net of tax		0.3	0.8
Other comprehensive (loss) earnings, net of tax		0.3	0.8
Ending Balance			(0.3)
Cumulative noncurrent deferred tax assets at end of period			0.2
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(143.6)	(129.1)	(130.7)
Other comprehensive (loss) earnings before reclassifications, net of Tax	(13.3)
Ending Balance	$ (145.8)	$ (143.6)	$ (129.1)