Supplemental cash flow information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash paid during the period for:
Interest	$ 34,606	$ 30,329	$ 40,930
Income taxes, net of refunds	16,681	19,838	$ 19,804
Increase (decrease) in accounts payable related to acquisition of property, plant, and equipment	$ (127)	$ 661