Accounting Policies - Reclassifications Out of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Other income (expense), net	$ (14.8)	$ (8.2)
Income tax expense	(38.0)	(18.5)
Net income (loss)	111.2	(4.0)
Reclassification out of Accumulated Other Comprehensive Income | Pension and post retirement benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Other income (expense), net	1.0	0.7
Income tax expense	(0.3)	(0.2)
Net income (loss)	0.7	0.5
Reclassification out of Accumulated Other Comprehensive Income | Amortization of initial net obligation and prior service cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Other income (expense), net	(0.4)	(0.4)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net loss
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income On Derivatives [Line Items]
Other income (expense), net	$ 1.4	$ 1.1