COMMITMENTS AND CONTINGENCIES - Environmental Matters (Details) - Meadowlark Midstream Gathering System $ in Thousands	6 Months Ended
Jun. 30, 2018 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrued environmental remediation, beginning balance	$ 5,344
Payments made	(1,805)
Additional accruals	1,600
Accrued environmental remediation, ending balance	$ 5,139

Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Amount of cash outflow to settle environmental loss contingencies.

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Increase (decrease) in the accrual for environmental loss contingencies during the reporting period.

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