ACCOUNTS RECEIVABLE, NET - Schedule of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounts receivable, net
Accounts receivable, gross	$ 25,242	$ 27,994
Less: Allowance for credit losses	(5,943)	(5,581)	$ (4,472)
Total	19,299	22,413
Billed
Accounts receivable, net
Accounts receivable, gross	20,087	19,703
Unbilled
Accounts receivable, net
Accounts receivable, gross	$ 5,155	$ 8,291