Other Comprehensive (Loss) Income - Schedule of Reclassification From Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassification, net of tax	$ 1.7	$ (6.4)	$ (65.9)	$ 55.2
Investment and other income net	2.8	1.0	47.2	0.4
Net Realized Loss | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Selling, general and administrative expenses	2.8		2.8
Net Actuarial Loss | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Cost of sales; Selling, general and administrative expenses	1.9	7.8	5.4	23.5
Translation Adjustments
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	2.8		2.8
Reclassification, net of tax	2.8		2.8	0.0
Net Prior Service Credit | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Cost of sales; Selling, general and administrative expenses	(0.7)	(4.0)	(2.2)	(12.0)
Settlements | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Cost of sales; Selling, general and administrative expenses		1.6		98.5
Curtailments | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Cost of sales; Selling, general and administrative expenses		(19.7)		(19.7)
Amortization of Pension and Other Postretirement Benefits Plan Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	1.2	(14.3)	3.2	90.3
Income tax expense (benefit)	(0.5)	7.9	(1.1)	(35.1)
Reclassification, net of tax	0.7	$ (6.4)	2.1	$ 55.2
Available-for-Sale Securities, Net Realized Gain on Equity Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications before tax	(1.8)		(52.8)
Income tax expense (benefit)			18.0
Reclassification, net of tax	(1.8)		(70.8)
Available-for-Sale Securities, Net Realized Gain on Equity Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Investment and other income net	$ 1.8		$ 52.8