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FAIR VALUE MEASUREMENT - Carrying value (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
FAIR VALUE MEASUREMENT      
Impairment charges for property, plant and equipment $ 21,866 $ 30,968 $ 11,626
Carrying value of long-term borrowings 619,477 393,614  
Convertible notes $ 0 $ 127,400