EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - OPEB - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2024	Jan. 02, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Jan. 01, 2024	Sep. 30, 2023
Amounts amortized to net periodic costs:
Net actuarial loss	$ (1,255)		$ 0	$ (1,751)	$ 0
Regulatory Assets
Regulatory Assets and Accumulated Other Comprehensive Income (Loss)
Regulatory Assets, Balance at beginning of period		$ 30,046		30,046
Net actuarial loss		12,706
Prior service cost		(61,244)				$ (61,244)	$ 0
Amounts amortized to net periodic costs:
Net actuarial loss		(355)
Regulatory Assets, Balance at end of period		(18,847)
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		$ 5,498
Net actuarial loss		7,027
Prior service cost		(20,039)				$ (20,039)	$ 0
Amounts amortized to net periodic costs:
Net actuarial loss		(141)
Accumulated Other Comprehensive Income (Loss), Balance at end of period		$ (7,655)