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REVENUE - Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Change in contract liability      
Beginning balance - Contract liability $ 3,509 $ 5,942  
Cash received in advance and not recognized as revenue 5,259 4,852  
Amounts recognized as revenue as performance obligations satisfied (2,500) (8,262)  
Ending balance - Contract liability 6,268 $ 2,532  
Accrued expenses and other liabilities      
Contract liabilities      
Contract liabilities $ 6,268   $ 3,509