EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (8,745)	$ (11,446)	$ (10,424)
Prior service (cost) credit	(101)	(102)	(102)
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	(5,684)	(7,909)	(7,442)
Prior service (cost) credit	144	179	197
Regulatory Assets | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	56,187	103,564
Amounts arising during the period, Net actuarial (gain)	(14,922)	(39,006)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(5,843)	(8,269)
Prior service (cost) credit	(101)	(102)
Regulatory Assets, Balance at end of period	35,321	56,187	103,564
Regulatory Assets | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	60,335	83,301
Amounts arising during the period, Net actuarial (gain)	(35,781)	(16,286)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(4,577)	(6,846)
Prior service (cost) credit	133	166
Regulatory Assets, Balance at end of period	20,110	60,335	83,301
Accumulated Other Comprehensive Income (Loss) | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	22,790	33,004
Amounts arising during the period, Net actuarial (gain)	(14,885)	(7,036)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(2,902)	(3,178)
Prior service (cost) credit	0	0
Accumulated Other Comprehensive Income (Loss), Balance at end of period	5,003	22,790	33,004
Accumulated Other Comprehensive Income (Loss) | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	12,696	13,823
Amounts arising during the period, Net actuarial (gain)	(18,422)	(76)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(1,107)	(1,064)
Prior service (cost) credit	11	13
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ (6,822)	$ 12,696	$ 13,823