Income taxes - Components of Net Deferred Tax Liability (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Tax effect of temporary differences related to:
Inventories	$ 3.6	$ 8.4
Marketable securities		2.2
Accrued OPEB costs	6.9	6.8
Accrued pension costs	33.3	16.8
Accrued environmental liabilities and other deductible differences	43.1	38.8
Tax loss and tax credit carryforwards	153.0	160.0
Valuation allowance	(0.2)	(1.3)
Adjusted gross deferred tax assets (liabilities)	239.7	231.7
Netting of items by tax jurisdiction	(109.8)	(80.2)
Total	129.9	151.5
Less net current deferred tax asset (liability)	9.6	18.8
Net noncurrent deferred tax asset (liability)	120.3	132.7
Tax effect of temporary differences related to:
Inventories	(10.3)	(6.6)
Marketable securities	(164.8)	(142.4)
Property and equipment	(104.5)	(89.3)
Other taxable differences	(30.3)	(25.3)
Investments in subsidiaries and affiliates	(258.3)	(258.2)
Tax on unremitted earnings of non-U.S. subsidiaries	(7.6)	(22.0)
Adjusted gross deferred tax assets (liabilities)	(575.8)	(543.8)
Netting of items by tax jurisdiction	109.8	80.2
Total	(466.0)	(463.6)
Less net current deferred tax asset (liability)	(11.2)	(6.4)
Net noncurrent deferred tax asset (liability)	$ (454.8)	$ (457.2)