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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Summary of Impact of Topic 606 Adoption on Unaudited Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Gathering services and related fees	$ 128,183	$ 101,792	$ 245,503	$ 237,597
Costs and expenses:
Cost of natural gas and NGLs	$ 24,384	$ 9,099	$ 44,670	$ 18,151
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,784	$ 28,688	$ 53,461	$ 57,257
Balances Without Adoption of Topic 606
Costs and expenses:
Cost of natural gas and NGLs	$ 27,559
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,472
Gathering Services and Related Fees
Revenues:
Gathering services and related fees	89,585	$ 84,801	$ 173,946	$ 202,814
Gathering Services and Related Fees \| Balances Without Adoption of Topic 606
Revenues:
Gathering services and related fees	86,892
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Costs and expenses:
Cost of natural gas and NGLs	$ (3,175)
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 312
Accounting Standard Update 2014-09 \| Gathering Services and Related Fees \| Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees	$ 2,693