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REVENUE - Revenue by Service (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disaggregation of Revenue [Line Items]      
Revenue $ 140,664 $ 103,968 $ 90,452
Online      
Disaggregation of Revenue [Line Items]      
Revenue 140,664 97,440 64,378
In-person      
Disaggregation of Revenue [Line Items]      
Revenue $ 0 $ 6,528 $ 26,074