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REVENUE - Contract Balances on Condensed Consolidated Balance Sheet (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Revenue from Contract with Customer [Abstract]      
Accounts receivable, net $ 8,123 $ 14,350  
Contract assets 1,183 1,359  
Deferred revenue $ (38,248) $ (32,908) $ (23,116)