SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details) $ in Thousands		1 Months Ended	12 Months Ended
	Aug. 28, 2025 USD ($)	Jul. 31, 2021 Megawatt	Sep. 30, 2025 USD ($) plan	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
Finite-Lived Intangible Assets [Line Items]
Number of megawatts | Megawatt		5
Operating revenues attributable to reportable segments			$ 2,036,412	$ 1,796,539	$ 1,962,994
Composite rate of depreciation			3.19%	2.69%	2.68%
Depreciation and amortization			$ 188,774	$ 166,567	$ 152,941
Debt instrument, term			7 years
Basis spread on variable rate			0.60%
SBC interest rate			4.58%	4.33%	4.79%
Deferred interest			$ 2,400	$ 2,600	$ 1,800
Loans receivable in other noncurrent assets			69,400	53,600
Proceeds from (payment for) other financing activity	$ 42,500
Notes receivable			$ 42,500	0
Number of noncontributory defined benefit retirement plans (in plans) | plan			2
Number of noncontributory medical and life insurance plans (in plans) | plan			2
Financial Asset, Not Past Due
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets			$ 21,500	18,100
Storage and Transportation Property, Base Gas | Nonutility
Finite-Lived Intangible Assets [Line Items]
Base gas cost			$ 21,400	21,400
Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term			1 year
Loans receivable term			5 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 attributable to reportable segments			$ 19,700	137,200	$ 48,500
Deferred revenue			$ 36,800	$ 22,300
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