SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Concentration of Risk (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Major customers
Operating revenue	$ 696,825	$ 648,872	$ 577,484	$ 564,087	$ 532,304	$ 498,564	$ 405,283	$ 368,590	$ 2,487,268	$ 1,804,741	$ 1,624,232
Operating revenues | Customer concentration | EOG Resources, Inc.
Major customers
Operating revenue									$ 258,194	$ 163,582	$ 124,262
Receivables | Credit concentration | EOG Resources, Inc.
Major customers
Concentration percentage									8.80%
Receivables | Credit concentration | Occidental Oil and Gas Corporation
Major customers
Concentration percentage									4.70%