REVENUE - TIMING OF REVENUE RECOGNITION (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2021 USD ($)
Timing of Revenue Recognition [Roll Forward]
Billed, beginning	$ 134,173
Unbilled, beginning	9,226
Customers' credit, beginning	25,934
Increase (decrease) for accounts receivable, billed	103,056
Increase (decrease) for unbilled revenue	32,715
Increase (decrease) for customers' credits	(5,171)
Billed, end	237,229
Unbilled, end	41,941
Customers' credit, end	$ 20,763