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FAIR VALUE MEASUREMENT - Carrying value (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
FAIR VALUE MEASUREMENT      
Impairment charges for property, plant and equipment $ 11,854 $ 21,866 $ 30,968
Carrying value of long-term borrowings 446,090 619,477  
Convertible notes $ 223,214 $ 0