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	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Gathering services and related fees	$ 127,479	$ 124,945	$ 372,982	$ 362,542
Costs and expenses:
Cost of natural gas and NGLs	$ 26,879	$ 18,177	$ 71,549	$ 36,328
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember	us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,743	$ 28,927	$ 80,204	$ 86,184
Balances Without Adoption of Topic 606
Costs and expenses:
Cost of natural gas and NGLs	$ 30,307		$ 81,468
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember		us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 26,373		$ 79,219
Gathering Services and Related Fees
Revenues:
Gathering services and related fees	86,427	$ 96,070	260,373	$ 298,884
Gathering Services and Related Fees \| Balances Without Adoption of Topic 606
Revenues:
Gathering services and related fees	83,351		255,546
Accounting Standard Update 2014-09 \| Effect of Change Increase / (Decrease)
Costs and expenses:
Cost of natural gas and NGLs	$ (3,428)		$ (9,919)
Type of Cost, Good or Service [Extensible List]	us-gaap:OilAndGasPurchasedMember		us-gaap:OilAndGasPurchasedMember
Depreciation and amortization	$ 370		$ 985
Accounting Standard Update 2014-09 \| Gathering Services and Related Fees \| Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees	$ 3,076		$ 4,827

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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