REVENUE RECOGNITION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Midstream services revenue — affiliate	$ 43,803	$ 34,009	$ 24,139	$ 33,847	$ 26,697	$ 25,437	$ 12,517	$ 12,099	$ 135,798	$ 76,750	$ 15,142
Gas gathering and compression
Disaggregation of Revenue [Line Items]
Midstream services revenue — affiliate									17,077	7,656	820
Gas processing
Disaggregation of Revenue [Line Items]
Midstream services revenue — affiliate									101,199	53,108	11,037
Transmission
Disaggregation of Revenue [Line Items]
Midstream services revenue — affiliate									15,942	15,848	3,285
NGL transmission
Disaggregation of Revenue [Line Items]
Midstream services revenue — affiliate									$ 1,580	$ 138	$ 0