DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue timing (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue recognition	$ 57,252	$ 52,979	$ 48,470	$ 49,774	$ 47,966	$ 51,337	$ 54,357	$ 52,887	$ 208,475	$ 206,547	$ 201,576
Performance obligations satisfied in prior periods									9,900
Contract with Customer, Asset, Net					100					100
Deferred revenue	$ 80				$ 451				80	451
Revenue recognized									300	100
Performance obligations transferred at a point in time [Member]
Revenue recognition									206,617	205,422	200,557
Performance obligations transferred over time [Member]
Revenue recognition									$ 1,858	$ 1,125	$ 1,019