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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]      
Balance at beginning of year $ 670 $ 665 $ 869
Expected losses 362 282 63
Write-offs, net of recoveries (496) (277) (267)
Balance at end of year $ 536 $ 670 $ 665