Commitments and contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Environmental Remediation Obligations [Abstract]
Balance at the beginning of the year	$ 50.2
Additions charged to expense, net	1.2
Payments, net	(0.8)
Changes in currency exchange rates	(0.1)
Balance at the end of period	50.5
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities	6.9	7.6
Noncurrent liabilities	43.6	42.6
Total	$ 50.5