Accounts Payable and Accrued Liabilities (Schedule of Accounts Payable and Accrued Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts Payable and Accrued Liabilities [Abstract]
Accounts payable	$ 6,924	$ 4,008
Goods received not yet invoiced	5,005	5,574
Accrued property taxes	2,782	2,751
Workers' compensation	3,800	2,969
Other	7,274	5,813
Total accounts payable and accrued liabilities	$ 25,785	$ 21,115