SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Revenues from Individual Customers Constituting 10% or More of Total Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Concentration Risk [Line Items]
Operating revenues	$ 208,267	$ 317,364	$ 633,639	$ 614,657	$ 649,050	$ 687,974	$ 720,868	$ 740,598	$ 1,773,927	$ 2,798,490	$ 2,487,268
North America Solutions
Concentration Risk [Line Items]
Operating revenues									$ 1,474,380	2,426,191	$ 2,093,601
North America Solutions | EOG Resources, Inc. | Total Revenues | Customer Concentration Risk
Concentration Risk [Line Items]
Operating revenues										$ 258,194