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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of presentation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES      
Net (loss) / income $ 1,058 $ (59,602) $ (9,548)
Net cash provided by / (used in) operating activities $ 1,882 $ (29,868) $ (14,831)