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SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Investments in affiliates, Impairment of long-lived assets, Asset retirement obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
SUMMARY OF PRINCIPAL ACCOUNTING POLICIES      
Impairment of long-lived assets $ 0 $ 5,738 $ 3,686
Impairment loss of property, plant and equipment 21,866 30,968 $ 11,626
Asset retirement obligation $ 26 $ 43