EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 31, 2017
Equity Method Investment [Roll Forward]
Investment in equity method investees, March 31, 2018	$ 682,227	$ 682,227
March cash distributions		9,644	$ 8,944
Investment in equity method investees, net of basis difference, February 28, 2018		690,485
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, March 31, 2018	682,227	$ 682,227
March cash distributions	2,568
Basis difference	(126,846)
Investment in equity method investees, net of basis difference, February 28, 2018	$ 557,949