REVENUE - TIMING OF REVENUE RECOGNITION (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2020 USD ($)
Timing of Revenue Recognition [Roll Forward]
Billed, beginning	$ 139,263
Unbilled, beginning	6,510
Customers' credit, beginning	27,116
Increase (decrease) for accounts receivable, billed	56,420
Increase (decrease) for unbilled revenue	34,651
Increase (decrease) for customers' credits	(7,788)
Billed, end	195,683
Unbilled, end	41,161
Customers' credit, end	$ 19,328