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FAIR VALUE MEASUREMENTS - Summary of changes in Level 3 instruments (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Balance at January $ 67 $ 128
Interest accrued on face value of 5% Notes 924 878
Conversion of 5% Notes 0 0
Change in fair value of Level 3 liabilities (801) (939)
Balance at December $ 190 $ 67