Supplemental cash flow information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash paid during the period for:
Interest	$ 30,329	$ 40,930	$ 27,828
Income taxes, net of refunds	19,838	19,804	$ 18,306
Increase (decrease) in accounts payable related to acquisition of property, plant, and equipment	$ 661	$ (223)