SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details)	1 Months Ended	12 Months Ended
	Jul. 31, 2021 Megawatt	Sep. 30, 2023 USD ($) plan	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)
Finite-Lived Intangible Assets [Line Items]
Number of megawatts | Megawatt	5
Operating revenues		$ 1,962,994,000	$ 2,905,979,000	$ 2,156,613,000
Composite rate of depreciation		2.68%	2.66%	2.42%
Depreciation and amortization		$ 152,941,000	$ 129,249,000	$ 111,387,000
SBC interest rate		4.79%	3.85%	1.68%
Deferred interest		$ 1,800,000	$ 900,000	$ 300,000
Loans receivable in other noncurrent assets		39,000,000	34,700,000
Gain (loss) amortized in earning		200,000	200,000
Amounts reclassified from accumulated other comprehensive income, net of tax		1,217,000	4,122,000
Intangible assets, acquired wholesale natural gas energy		$ 100,000
Number of noncontributory defined benefit retirement plans (in plans) | plan		2
Number of noncontributory medical and life insurance plans (in plans) | plan		2
Pension
Finite-Lived Intangible Assets [Line Items]
Employer discretionary contributions		$ 0	0
Employer contributions		579,000	628,000
OPEB
Finite-Lived Intangible Assets [Line Items]
Employer contributions		4,192,000	6,082,000
Cash Flow Hedges
Finite-Lived Intangible Assets [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of tax		1,053,000	1,054,000
Financial Asset, Not Past Due
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets		$ 15,100,000	14,500,000
Seven-Year Treasury Rate
Finite-Lived Intangible Assets [Line Items]
Debt instrument, term		7 years
Basis spread on variable rate		0.60%
Storage and Transportation Property, Base Gas | Unregulated
Finite-Lived Intangible Assets [Line Items]
Base gas cost		$ 20,900,000	15,100,000
Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term		1 year
Loans receivable term		2 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		$ 48,500,000	53,000,000
Deferred revenue		$ 58,700,000	$ 33,800,000
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