AMORTIZING INTANGIBLE ASSETS - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 463,103	$ 468,852
Accumulated amortization	(219,330)	(195,436)
Net	243,773	273,416
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(14,891)	(13,905)
Net	9,304	10,290
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(163,152)	(143,962)
Net	115,296	134,486
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	160,460	166,209
Accumulated amortization	(41,287)	(37,569)
Net	$ 119,173	$ 128,640