EQUITY METHOD INVESTMENTS - Rollforward of the Investment in Equity Method Investees (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	$ 670,827	$ 670,827
June cash distributions		17,124	$ 18,003
Investment in equity method investees, net of basis difference, May 31, 2018		690,485
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, June 30, 2018	670,827	$ 670,827
June cash distributions	2,580
Basis difference	(123,508)
Investment in equity method investees, net of basis difference, May 31, 2018	$ 549,899