Inventories (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Inventories
Finished goods	$ 8,505	$ 14,550
Work-in-process	16,522	13,088
Raw materials and purchased parts	117,789	104,488
Stores and supplies	30,197	24,865
Total inventories	173,013	156,991
Valued at lower of cost or market:
LIFO basis	107,162	101,465
FIFO basis	17,655	7,473
Average cost basis	48,196	48,053
Total inventories	173,013	156,991
Excess of inventories valued at current costs over LIFO	23,000	23,000
Change in income as a result of LIFO basis inventory valuation over FIFO basis valuation	$ 0	$ 0	$ 600