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INCOME TAXES - Rollforward of uncertain tax position (Details) - Research Tax Credit Carryforward [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Uncertain tax positions    
Uncertain tax positions at December 31 $ 998 $ 998
Increase due to positions taken in prior periods 10 0
Uncertain tax positions at December 31 $ 1,008 $ 998