EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity Method Investment [Roll Forward]
Investment in Ohio Gathering, September 30, 2019	$ 648,748	$ 648,748		$ 648,748
September cash distributions				28,008	$ 26,528
Impairment loss		677	$ 1,169	1,197	$ 3,703
Investment in Ohio Gathering, net of basis difference, August 31, 2019				649,250
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in Ohio Gathering, September 30, 2019	620,496	$ 620,496		$ 620,496
September cash distributions	3,478
Impairment loss	757
Basis difference	(106,818)
Investment in Ohio Gathering, net of basis difference, August 31, 2019	$ 517,913