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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, 2020
Dec. 31, 2019
Dec. 31, 2018
SUMMARY OF PRINCIPAL ACCOUNTING POLICIES      
Impairment loss of investment $ 24,060 $ 0 $ 5,738
Impairment loss of property, plant and equipment 11,854 21,866 $ 30,968
Asset retirement obligation $ 0 $ 0