REVENUE - TIMING OF REVENUE RECOGNITION (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019 USD ($)
Timing of Revenue Recognition [Roll Forward]
Billed, beginning	$ 205,490
Unbilled, beginning	7,199
Customers' credit, beginning	27,325
Increase (decrease) for accounts receivable, billed	68,434
Increase (decrease) for unbilled revenue	35,651
Increase (decrease) for customers' credits	(9,163)
Billed, end	273,924
Unbilled, end	42,850
Customers' credit, end	$ 18,162