EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Pension
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	$ (117)	$ (300)	$ (8,745)
Prior service (cost) credit	(61)	(103)	(101)
Pension | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	(2,407)	(10,493)
Amounts arising during the period, Prior service (credit)	0
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	2	(87)
Prior service (cost) credit	(61)	(103)
Pension | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	934	(4,048)
Amounts arising during the period, Prior service (credit)	0
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	(119)	(213)
Prior service (cost) credit	0	0
Pension | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	24,638	35,321
Amounts amortized to net periodic costs:
Regulatory Assets, Balance at end of period	22,172	24,638	35,321
Pension | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	742	5,003
Amounts amortized to net periodic costs:
Accumulated Other Comprehensive Income (Loss), Balance at end of period	1,557	742	5,003
OPEB
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	(4,262)	0	(5,684)
Prior service (cost) credit	10,013	0	144
OPEB | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	27,108	9,936
Amounts arising during the period, Prior service (credit)	(60,504)
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	(3,098)	0
Prior service (cost) credit	7,458	0
OPEB | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	14,080	12,320
Amounts arising during the period, Prior service (credit)	(19,376)
Amounts amortized to net periodic costs:
Net actuarial gain (loss)	(1,164)	0
Prior service (cost) credit	2,555	0
OPEB | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	30,046	20,110
Amounts amortized to net periodic costs:
Regulatory Assets, Balance at end of period	1,010	30,046	20,110
OPEB | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	5,498	(6,822)
Amounts amortized to net periodic costs:
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 1,593	$ 5,498	$ (6,822)