SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2021 Megawatt	Sep. 30, 2024 USD ($) plan	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Finite-Lived Intangible Assets [Line Items]
Number of megawatts | Megawatt	5
Operating revenues		$ 1,796,539	$ 1,962,994	$ 2,905,979
Composite rate of depreciation		2.69%	2.68%	2.66%
Depreciation and amortization		$ 166,567	$ 152,941	$ 129,249
Debt instrument, term		7 years
Basis spread on variable rate		0.60%
SBC interest rate		4.33%	4.79%	3.85%
Deferred interest		$ 2,600	$ 1,800	$ 900
Loans receivable in other noncurrent assets		53,600	39,000
Amounts reclassified from accumulated other comprehensive income, net of tax		$ 78	1,217
Number of noncontributory defined benefit retirement plans (in plans) | plan		2
Number of noncontributory medical and life insurance plans (in plans) | plan		2
Cash Flow Hedges
Finite-Lived Intangible Assets [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of tax		$ 1,054	1,053
Financial Asset, Not Past Due
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets		18,100	15,100
Storage and Transportation Property, Base Gas | Nonutility
Finite-Lived Intangible Assets [Line Items]
Base gas cost		$ 21,400	20,900
Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term		1 year
Loans receivable term		3 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term		10 years
Loans receivable term		10 years
ES
Finite-Lived Intangible Assets [Line Items]
Operating revenues		$ 137,200	48,500
Deferred revenue		$ 22,300	$ 58,700
ES | Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term		1 year
ES | Maximum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term		10 years