Supplemental cash flow information (Schedule of Supplemental cash flow) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Supplemental Cash Flow Information [Abstract]
Interest paid, net of interest capitalized	$ 16,524	$ 19,024
Interest received	1,265	1,918
Net income taxes paid	13,850	18,455
Non-cash purchase of property, plant and equipment under capital lease	5,930	8,335
Non-cash right of use assets obtained (reassessed) in exchange for new lease obligations	$ (139)	$ 20,179