EMPLOYEE BENEFIT PLANS - ASSUMED HCCTR (Details) - OPEB - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
HCCTR	8.30%	8.50%	6.70%
Ultimate HCCTR	4.50%	4.50%	4.80%
Year ultimate HCCTR reached	2025	2025	2022
Effect of a 1 percentage point increase in the HCCTR on:
Year-end benefit obligation	$ 32,019	$ 28,803	$ 26,025
Total service and interest cost	2,468	2,331	2,026
Effect of a 1 percentage point decrease in the HCCTR on:
Year-end benefit obligation	(25,466)	(22,862)	(20,427)
Total service and interest costs	$ (1,909)	$ (1,801)	$ (1,593)