Property, plant and equipment - Schedule of Property,Plant and Equipment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Cost	$ 130,460	$ 114,308
Accumulated depreciation	(76,625)	(68,171)
Total	53,835	46,137
Buildings [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Cost	3,911	3,829
Plant, equipment and instrumentation [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Cost	120,982	104,782
Furniture and fixtures [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Cost	$ 5,567	$ 5,697