Commitments and contingencies - Changes in Accrued Environmental Remediation (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Commitments And Contingent Liabilities [Line Items]
Balance at the beginning of the year	$ 43.2
Additions charged to expense, net	14.2
Payments, net	(6.3)
Balance at the end of the period	51.1
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities	8.0	8.6
Noncurrent liabilities	43.1	34.6
Total	$ 51.1