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REVENUE - Roll Forward of Deferred Revenue (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Change in Contract with Customer, Liability [Roll Forward]    
Balance at beginning of period $ 32,908 $ 23,116
Additions 29,857 37,200
Revenue recognized (24,517) (27,408)
Balance at end of period $ 38,248 $ 32,908