BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable and Inventories (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Activity in the allowance for doubtful accounts
Beginning balance	$ 405	$ 530
Additional reserves	91	(5)	$ 192
Writeoffs	(123)	(132)
Effect of foreign currency translation	9	12
Ending balance	382	405	$ 530
Inventories
Parts and raw materials	44,750	35,849
Work-in-process	6,186	6,951
Finished goods	12,042	10,585
Inventories	$ 62,978	$ 53,385