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FAIR VALUE MEASUREMENT - Carrying value (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Impairment charges for property, plant and equipment $ 11,626 $ 22,757 $ 6,992
Carrying value of long-term borrowings 404,341 493,455  
Fair value of long-term borrowings 404,341 493,455  
Carrying value of convertible notes 126,476 125,569  
Idle assets      
Impairment charges for property, plant and equipment $ 11,626 $ 22,757 $ 6,992