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ASSET RETIREMENT OBLIGATIONS - Retirement obligation rollforward (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Changes in the asset retirement obligations    
Asset retirement obligation liability, beginning balance $ 34,000 $ 32,201
Settlements (866) (267)
Accretion of liability 991 1,901
Adjustment reflecting updated estimates 425 (54)
Foreign exchange revaluation 334 219
Asset retirement obligation liability, ending balance 34,884 34,000
Less current portion 4,883 3,232
Long-term portion $ 30,001 $ 30,768