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, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity Method Investment [Roll Forward]
Investment in equity method investees, September 30, 2018	$ 660,254	$ 660,254		$ 660,254
September cash distributions				26,528	$ 28,715
Impairment loss		1,169	$ (350)	3,703	$ 3,691
Investment in equity method investees, net of basis difference, August 31, 2018				690,485
OGC | Summit Midstream Partners, LLC
Equity Method Investment [Roll Forward]
Investment in equity method investees, September 30, 2018	660,254	$ 660,254		$ 660,254
September cash distributions	2,912
Impairment loss	1,837
Basis difference	(120,170)
Investment in equity method investees, net of basis difference, August 31, 2018	$ 544,833