Supplemental Cash Flow Information (Schedule of Supplemental Cash Flow) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]
Interest paid, net of interest capitalized	$ 23,360	$ 5,792	$ 4,989
Interest received	3,196	1,861	2,657
Net income taxes paid	28,281	54,037	34,661
Non-cash purchase of property, plant and equipment under capital lease	$ 8,820	$ 3,376	$ 943