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ASSET RETIREMENT OBLIGATIONS (Asset Retirement Obligations Operating Costs) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
ASSET RETIREMENT OBLIGATIONS [Abstract]        
Accretion $ 2,684 $ 2,857 $ 8,036 $ 8,620
Depreciation 1,512 1,494 4,192 4,741
Total $ 4,196 $ 4,351 $ 12,228 $ 13,361