Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
Raw materials	$ 13,258	$ 8,231
Supplies and service parts	23,627	21,841
Finished products	36,474	36,690
Inventory at FIFO cost	73,359	66,762
Excess of FIFO cost over LIFO cost	(4,483)	(4,483)
Total inventory, net	$ 68,876	$ 62,279