Significant Accounting Policies - Changes in Accumulated Other Comprehensive Loss Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 5,893.3	$ 5,353.3
Other comprehensive earnings (loss), net of tax	2.8	0.4
Ending Balance	5,926.7	5,299.4
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(158.1)	(144.9)
Amounts reclassified from accumulated other comprehensive earnings (loss), net of tax	2.5	2.5
Other comprehensive earnings (loss), net of tax	2.5	2.5
Ending Balance	(155.6)	(142.4)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(0.3)	(0.9)
Other comprehensive earnings (loss) before reclassifications, net of tax	0.3	(2.1)
Other comprehensive earnings (loss), net of tax	0.3	(2.1)
Ending Balance		(3.0)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(158.4)	(145.8)
Other comprehensive earnings (loss) before reclassifications, net of tax	0.3	(2.1)
Amounts reclassified from accumulated other comprehensive earnings (loss), net of tax	2.5	2.5
Other comprehensive earnings (loss), net of tax	2.8	0.4
Ending Balance	$ (155.6)	$ (145.4)