EMPLOYEE BENEFIT PLANS - ASSUMED HCCTR (Details) - OPEB - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
HCCTR	7.90%	8.30%	8.50%
Ultimate HCCTR	4.50%	4.50%	4.50%
Year ultimate HCCTR reached	2024	2025	2025
Effect of a 1 percentage point increase in the HCCTR on:
Year-end benefit obligation	$ 36,260	$ 32,019	$ 28,803
Total service and interest cost	2,482	2,468	2,331
Effect of a 1 percentage point decrease in the HCCTR on:
Year-end benefit obligation	(28,743)	(25,466)	(22,862)
Total service and interest costs	$ (1,937)	$ (1,909)	$ (1,801)