AMORTIZING INTANGIBLE ASSETS - Intangible Assets and Liabilities Subject to Amortization (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 461,505	$ 468,852
Accumulated amortization	(202,794)	(195,436)
Net	258,711	273,416
Favorable gas gathering contracts
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	24,195	24,195
Accumulated amortization	(14,294)	(13,905)
Net	9,901	10,290
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	278,448	278,448
Accumulated amortization	(150,180)	(143,962)
Net	128,268	134,486
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	158,862	166,209
Accumulated amortization	(38,320)	(37,569)
Net	$ 120,542	$ 128,640