Accounts Payable and Accrued Liabilities (Schedule of Accounts Payable and Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts Payable and Accrued Liabilities [Abstract]
Accounts payable	$ 4,008	$ 4,829
Goods received not yet invoiced	5,574	3,072
Accrued property taxes	2,751	2,992
Workers' compensation reserve and IBNR	2,969	2,658
Other	5,813	6,367
Total accounts payable and accrued liabilities	$ 21,115	$ 19,918