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COMMITMENTS AND CONTINGENCIES - Southbridge Landfill Environmental Remediation Liability (Details) - Southbridge Landfill - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Accrual for Environmental Loss Contingencies [Roll Forward] |  |
Beginning balance | $ 4,596 | $ 5,173
Accretion expense | 30 | 34
Obligations incurred | 27 | 0
Obligations settled | (12) | (141)
Ending balance | $ 4,641 | $ 5,066

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