EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Mar. 31, 2018
Equity Method Investment [Roll Forward]
Investment in equity method investees, March 31, 2019	$ 640,226	$ 640,226
March cash distributions		8,583	$ 9,644
Investment in equity method investees, net of basis difference, February 28, 2019		649,250
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, March 31, 2019	640,226	$ 640,226
March cash distributions	2,883
Basis difference	(113,493)
Investment in equity method investees, net of basis difference, February 28, 2019	$ 529,616