RECEIVABLES (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020		Dec. 31, 2019
Receivables [Abstract]
Trade accounts receivable	$ 172,664	$ 157,513
Unbilled receivables	40,604	46,471
Allowance for credit losses	(8,974)	(9,918)		$ (9,990)
Total	$ 204,294	$ 194,066	[1]

[1] Includes billed and unbilled amounts, net of allowance for credit losses. See Note 3 for details.