Inventories (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Inventories
Finished goods	$ 8,323	$ 7,749
Work-in-process	5,393	2,795
Raw materials and purchased parts	21,188	20,259
Stores and supplies	35,589	30,198
Total inventories	70,493	61,001
Valued at lower of cost or market:
LIFO basis	14,133	14,690
FIFO basis	7,567	5,485
Average cost basis	48,793	40,826
Total inventories	70,493	61,001
Excess of inventories valued at current costs over LIFO	13,000	13,000
Change in income as a result of LIFO basis inventory valuation over FIFO basis valuation	$ 0	$ 0