Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 13,502	$ 11,767
Supplies and service parts	21,822	21,475
Finished products	19,461	13,261
Inventory at FIFO cost	54,785	46,503
Excess of FIFO cost over LIFO cost	(5,734)	(5,734)
Total inventory, net	$ 49,051	$ 40,769