DEBT - CREDIT FACILITIES AND SHORT-TERM DEBT (Details) - USD ($)	12 Months Ended
	Sep. 30, 2024	Aug. 07, 2024	Sep. 30, 2023
Letter of Credit | NJR bank revolving credit facility
Short-term Debt [Line Items]
Letters of credit outstanding, amount	$ 12,300,000		$ 5,700,000
Letter of Credit | NJNG bank revolving credit facility
Short-term Debt [Line Items]
Letters of credit outstanding, amount	$ 700,000		700,000
Revolving Credit Facility | NJR bank revolving credit facility
Short-term Debt [Line Items]
Commitment fee percentage	0.10%
Revolving Credit Facility | NJNG bank revolving credit facility
Short-term Debt [Line Items]
Commitment fee percentage	0.075%
Revolving Credit Facility | Committed Credit Facilities Due August 2029 | NJR bank revolving credit facility
Short-term Debt [Line Items]
Total borrowing capacity	$ 575,000,000	$ 575,000,000
Loans outstanding	$ 236,700,000
Weighted average interest rate	6.23%
Remaining borrowing capacity	$ 325,951,000
Revolving Credit Facility | Committed Credit Facilities Due August 2029 | NJNG bank revolving credit facility
Short-term Debt [Line Items]
Total borrowing capacity	250,000,000	$ 250,000,000
Loans outstanding	$ 55,100,000
Weighted average interest rate	4.98%
Remaining borrowing capacity	$ 194,169,000
Revolving Credit Facility | Committed Credit Facilities Due September 2027 | NJR bank revolving credit facility
Short-term Debt [Line Items]
Total borrowing capacity			650,000,000
Loans outstanding			$ 217,300,000
Weighted average interest rate			6.53%
Remaining borrowing capacity			$ 426,967,000
Revolving Credit Facility | Committed Credit Facilities Due September 2027 | NJNG bank revolving credit facility
Short-term Debt [Line Items]
Total borrowing capacity			250,000,000
Loans outstanding			$ 34,800,000
Weighted average interest rate			5.48%
Remaining borrowing capacity			$ 214,469,000