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
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Gathering services and related fees	$ 84,361	$ 118,013
Costs and expenses:
Cost of natural gas and NGLs	20,286	9,052
Depreciation and amortization	26,677	$ 28,569
Balances Without Adoption of Topic 606
Revenues:
Gathering services and related fees	85,303
Costs and expenses:
Cost of natural gas and NGLs	23,602
Depreciation and amortization	26,374
Accounting Standard Update 2014-09 | Effect of Change Increase / (Decrease)
Revenues:
Gathering services and related fees	(942)
Costs and expenses:
Cost of natural gas and NGLs	(3,316)
Depreciation and amortization	$ 303