Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash paid during the year:
Interest paid, net of amounts capitalized	$ 13,706	$ 16,178	$ 14,667
Non-cash investing activities - capital assets acquired under installment contracts	$ 1,309	$ 90	$ 6,005