EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Pension
Amounts amortized to net periodic costs:
Net actuarial (loss)	$ (1,203)	$ (117)	$ (300)
Prior service credit	0	(61)	(103)
Pension | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	(13,368)	(2,407)
Amounts arising during the period, Prior service (credit)		0
Amounts amortized to net periodic costs:
Net actuarial (loss)	(994)	2
Prior service credit	0	(61)
Pension | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	(3,303)	934
Amounts arising during the period, Prior service (credit)		0
Amounts amortized to net periodic costs:
Net actuarial (loss)	(209)	(119)
Prior service credit	0	0
Pension | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	22,172	24,638
Amounts amortized to net periodic costs:
Regulatory Assets, Balance at end of period	7,810	22,172	24,638
Pension | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	1,557	742
Amounts amortized to net periodic costs:
Accumulated Other Comprehensive Income (Loss), Balance at end of period	(1,955)	1,557	742
OPEB
Amounts amortized to net periodic costs:
Net actuarial (loss)	(7,171)	(4,262)	0
Prior service credit	13,080	10,013	0
OPEB | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	27,178	27,108
Amounts arising during the period, Prior service (credit)		(60,504)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(5,114)	(3,098)
Prior service credit	9,801	7,458
OPEB | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Amounts arising during the period, Net actuarial (gain) loss	10,086	14,080
Amounts arising during the period, Prior service (credit)		(19,376)
Amounts amortized to net periodic costs:
Net actuarial (loss)	(2,057)	(1,164)
Prior service credit	3,279	2,555
OPEB | Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period	1,010	30,046
Amounts amortized to net periodic costs:
Regulatory Assets, Balance at end of period	32,875	1,010	30,046
OPEB | Accumulated Other Comprehensive Income (Loss)
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period	1,593	5,498
Amounts amortized to net periodic costs:
Accumulated Other Comprehensive Income (Loss), Balance at end of period	$ 12,901	$ 1,593	$ 5,498