Property, plant and equipment - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation	$ 2.3	$ 1.7	$ 1.6
Cost of Goods Sold, Depreciation	$ 2.0	$ 1.7	$ 1.5