AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 464,982	$ 462,629
Accumulated amortization	(178,317)	(161,284)
Net	286,665	301,345
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(13,128)	(12,350)
Net	11,067	11,845
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(130,891)	(117,821)
Net	147,557	160,627
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	162,339	159,986
Accumulated amortization	(34,298)	(31,113)
Net	128,041	128,873
Unfavorable Gas Gathering Contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,962	10,962
Accumulated amortization	(10,153)	(9,074)
Net	$ 809	$ 1,888