REVENUE - TIMING OF REVENUE RECOGNITION (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2019 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	111,156
Increase (decrease) for unbilled revenue	57,146
Increase (decrease) for customers' credits	2,457
Billed, end	250,419
Unbilled, end	63,656
Customers' credit, end	$ 29,573