Inventories (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 11,998	$ 11,767
Supplies and service parts	22,213	21,475
Finished products	16,993	13,261
Inventory at FIFO cost	51,204	46,503
Excess of FIFO cost over LIFO cost	(3,005)	(5,734)
Total inventory, net	$ 48,199	$ 40,769