Significant Accounting Policies - Changes in Accumulated Other Comprehensive Loss Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 5,493.6	$ 5,097.3	$ 5,353.3	$ 4,949.4
Other comprehensive earnings (loss), net of tax	3.6	2.5	7.2	8.8
Ending Balance	5,758.9	5,304.4	5,758.9	5,304.4
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(140.0)	(136.0)	(144.9)	(141.5)
Amounts reclassified from accumulated other comprehensive earnings (loss), net of tax	3.1	2.7	8.0	8.2
Other comprehensive earnings (loss), net of tax	3.1	2.7	8.0	8.2
Ending Balance	(136.9)	(133.3)	(136.9)	(133.3)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(2.2)	(1.3)	(0.9)	(2.1)
Other comprehensive earnings (loss) before reclassifications, net of tax	0.5	(0.2)	(0.8)	0.6
Other comprehensive earnings (loss), net of tax	0.5	(0.2)	(0.8)	0.6
Ending Balance	(1.7)	(1.5)	(1.7)	(1.5)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(142.2)	(137.3)	(145.8)	(143.6)
Other comprehensive earnings (loss) before reclassifications, net of tax	0.5	(0.2)	(0.8)	0.6
Amounts reclassified from accumulated other comprehensive earnings (loss), net of tax	3.1	2.7	8.0	8.2
Other comprehensive earnings (loss), net of tax	3.6	2.5	7.2	8.8
Ending Balance	$ (138.6)	$ (134.8)	$ (138.6)	$ (134.8)