AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 468,088	$ 462,629
Accumulated amortization	(186,881)	(161,284)
Net	281,207	301,345
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(13,517)	(12,350)
Net	10,678	11,845
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(137,426)	(117,821)
Net	141,022	160,627
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	165,445	159,986
Accumulated amortization	(35,938)	(31,113)
Net	129,507	128,873
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	(10,692)	(9,074)
Net	$ 270	$ 1,888