FINANCING RECEIVABLES - Components of Receivables (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Less: allowance for expected credit losses	$ (829)	$ (1,489)	$ (2,971)
Total financing receivables	15,748	22,298
Long-Term Financing Arrangement
Financing Receivable, Recorded Investment, Past Due [Line Items]
Long-term financing arrangements, gross	17,952	24,082
Less: allowance for expected credit losses	(818)	(1,390)
Less: unearned income	(1,600)	(2,268)
Total financing receivables	$ 15,534	$ 20,424