Cash Flow Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash paid during the year:
Interest paid, net of amounts capitalized	$ 3,391	$ 3,537
Non-cash investing activities - capital assets acquired under installment contracts		617
Non-cash financing activities - common shares issued for performance based compensation, net of withholding taxes paid		400
Restricted cash included in other assets
Cash and cash equivalents	19,191		$ 40,759
Restricted cash included in other assets	250		250
Total cash, cash equivalents and restricted cash	$ 19,441	$ 15,804	$ 41,009	$ 21,146