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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Contract liabilities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Contract liabilities:    
Deferred revenue, Balance at Beginning of Period $ 3,099 $ 2,932
Deferred revenue, Billings 5,006 5,564
Deferred revenue, Revenue Recognized (5,268) (5,397)
Deferred revenue, Balance at End of Period $ 2,837 $ 3,099