Property, plant and equipment - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation	$ 4.2	$ 2.3	$ 1.7
Product [Member]
Property, Plant and Equipment [Line Items]
Cost, Depreciation	$ 2.4	$ 2.0	$ 1.7