Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 29,150	$ 30,166
Work in process	7,127	4,981
Supplies and service parts	46,049	45,366
Finished products	26,497	21,765
Inventory at FIFO cost	108,823	102,278
Excess of FIFO cost over LIFO cost	(12,599)	(12,599)
Total inventory, net	$ 96,224	$ 89,679