Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Current Liabilities [Line Items]
Payroll and incentive compensation	$ 9,146	$ 16,101
Union dues and benefits	12,494	10,261
Profit sharing and thrift plan	443	2,004
Taxes, other than income taxes	6,807	7,639
Other	6,148	6,377
Other current liabilities, total	$ 35,038	$ 42,382