Accrued Liabilities - (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Current Liabilities [Line Items]
Payroll and incentive compensation	$ 21,641	$ 9,146
Net asset adjustments	11,210	0
Union dues and benefits	11,465	12,494
Profit sharing and thrift plan	1,215	443
Taxes, other than income taxes	7,999	6,807
Other	7,828	4,843
Other current liabilities, total	$ 61,358	$ 33,733