Property, plant and equipment - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Depreciation expense	$ 15.7	$ 14.3	$ 13.6