EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (300)	$ (8,745)	$ (11,446)
Prior service (cost) credit	(103)	(101)	(102)
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	0	(5,684)	(7,909)
Prior service (cost) credit	0	144	179
Regulatory Assets | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	35,321	56,187
Amounts arising during the period, Net actuarial (gain) loss	(10,493)	(14,922)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(87)	(5,843)
Prior service (cost) credit	(103)	(101)
Regulatory Assets, Balance at end of period	24,638	35,321	56,187
Regulatory Assets | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	20,110	60,335
Amounts arising during the period, Net actuarial (gain) loss	9,936	(35,781)
Amounts amortized to net periodic costs:
Net actuarial (loss)	0	(4,577)
Prior service (cost) credit	0	133
Regulatory Assets, Balance at end of period	30,046	20,110	60,335
Accumulated Other Comprehensive Income (Loss) | Pension
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	5,003	22,790
Amounts arising during the period, Net actuarial (gain) loss	(4,048)	(14,885)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(213)	(2,902)
Prior service (cost) credit	0	0
Accumulated Other Comprehensive Income (Loss), Balance at end of period	742	5,003	22,790
Accumulated Other Comprehensive Income (Loss) | OPEB
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	(6,822)	12,696
Amounts arising during the period, Net actuarial (gain) loss	12,320	(18,422)
Amounts amortized to net periodic costs:
Net actuarial (loss)	0	(1,107)
Prior service (cost) credit	0	11
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 5,498	$ (6,822)	$ 12,696