ACCOUNTS RECEIVABLE - Allowance for doubtful accounts (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
ACCOUNTS RECEIVABLE
Balance at beginning of period	$ 500	$ 199	$ 199
Provision for uncollectible accounts	(316)	$ (333)	377
Uncollectible accounts written off			(76)
Uncollectible accounts written off	195
Balance at end of period	$ 379		$ 500