SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ADDITIONAL INFORMATION (Details)	12 Months Ended
	Sep. 30, 2022 USD ($) plan	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Operating revenues	$ 2,905,979,000	$ 2,156,613,000	$ 1,953,668,000
Investment tax credit, solar property, percentage				26.00%
Composite rate of depreciation	2.66%	2.42%	2.65%
Depreciation and amortization	$ 129,249,000	$ 111,387,000	$ 107,368,000
SBC interest rate	3.85%	1.68%	1.97%
Deferred interest	$ 857,000	$ 346,000	$ 511,000
Loans receivable in other noncurrent assets	34,700,000	32,300,000
Gain (loss) amortized in earning	219,000	223,000
Amounts reclassified from accumulated other comprehensive income, net of tax	4,122,000	4,293,000
Intangible assets, acquired wholesale natural gas energy	2,300,000
Proceeds from sale leaseback transaction	17,300,000	0	4,000,000
Proceeds from sale leaseback transactions - solar	$ 24,071,000	17,673,000	42,927,000
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	628,000	548,000
OPEB
Finite-Lived Intangible Assets [Line Items]
Employer contributions	6,082,000	7,198,000
Clean Energy Ventures
Finite-Lived Intangible Assets [Line Items]
Proceeds from sale leaseback transactions - solar	24,100,000	17,700,000	42,900,000
NJNG
Finite-Lived Intangible Assets [Line Items]
Proceeds from sale leaseback transaction	17,300,000	0	$ 4,000,000
Cash Flow Hedges
Finite-Lived Intangible Assets [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of tax	1,054,000	1,021,000
Financial Asset, Not Past Due
Finite-Lived Intangible Assets [Line Items]
Loans receivable in other current assets	$ 14,500,000	14,200,000
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	$ 15,100,000	$ 7,900,000
Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term	1 year
Loans receivable term	2 years
Minimum | Clean Energy Ventures
Finite-Lived Intangible Assets [Line Items]
Term of lease	5 years
Minimum | NJNG
Finite-Lived Intangible Assets [Line Items]
Term of lease	7 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term	10 years
Loans receivable term	10 years
Maximum | Clean Energy Ventures
Finite-Lived Intangible Assets [Line Items]
Term of lease	15 years
Maximum | NJNG
Finite-Lived Intangible Assets [Line Items]
Term of lease	11 years
Energy Services
Finite-Lived Intangible Assets [Line Items]
Operating revenues	$ 53,000,000
Deferred revenue	$ 33,800,000
Energy Services | Minimum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term	1 year
Energy Services | Maximum
Finite-Lived Intangible Assets [Line Items]
Storage and pipeline capacity, contract term	10 years