Property, plant and equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Estimated costs for completion of capital additions under construction	$ 102,836
Depreciation and depletion expense	$ 186,540	$ 188,533	$ 178,049