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COMMITMENTS AND CONTINGENCIES - Environmental Remediation Liability (Details) - Southbridge Landfill - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Accrual for Environmental Loss Contingencies [Roll Forward]    
Beginning balance $ 4,261 $ 4,596
Accretion expense 56 62
Obligations incurred 0 28
Revision in estimates 0 (188)
Obligations settled (281) (293)
Ending balance $ 4,036 $ 4,205