Accounts Payable and Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts Payable and Accrued Liabilities [Abstract]
Trade accounts payable	$ 169,293	$ 128,106
Accrued wages, salaries and employee benefits	41,979	49,871
Accrued interest	12,723	12,149
Accrued discounts and allowances	7,283	10,291
Accrued utilities	6,759	6,712
Accrued taxes other than income taxes payable	6,907	6,946
Other	11,677	9,624
Accounts payable and accrued liabilities	$ 256,621	$ 223,699