Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Income tax refunds received	$ 5,031	$ 3,838	$ 4,068
Income taxes paid	(35,893)	(26,587)	(24,960)
Interest paid, net of amount capitalized	$ (79,047)	$ (87,076)	$ (91,496)