EMPLOYEE BENEFIT PLANS - ASSUMED HCCTR (Details) - OPEB - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
HCCTR	7.60%	7.90%	8.30%
Ultimate HCCTR	4.50%	4.50%	4.50%
Effect of a 1 percentage point increase in the HCCTR on:
Year-end benefit obligation	$ 49,061	$ 36,260	$ 32,019
Total service and interest cost	2,923	2,482	2,468
Effect of a 1 percentage point decrease in the HCCTR on:
Year-end benefit obligation	(38,747)	(28,743)	(25,466)
Total service and interest costs	$ (2,250)	$ (1,937)	$ (1,909)