DEBT - CREDIT FACILITIES AND SHORT-TERM DEBT (Details) - USD ($)	6 Months Ended
	Mar. 31, 2025	Sep. 30, 2024
Letter of Credit on Behalf of NJRES | NJR
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 21,400,000	$ 12,300,000
Letter of Credit | NJNG
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 700,000	700,000
Revolving Credit Facility | NJR
Line of Credit Facility [Line Items]
Commitment fee percentage	0.10%
Revolving Credit Facility | NJNG
Line of Credit Facility [Line Items]
Commitment fee percentage	0.075%
Revolving Credit Facility | Committed Credit Facilities Due August 2029 | NJR
Line of Credit Facility [Line Items]
Total borrowing capacity	$ 575,000,000	575,000,000
Loans outstanding	$ 162,450,000	$ 236,700,000
Weighted average interest rate	5.56%	6.23%
Remaining borrowing capacity	$ 391,146,000	$ 325,951,000
Revolving Credit Facility | Committed Credit Facilities Due August 2029 | NJNG
Line of Credit Facility [Line Items]
Total borrowing capacity	250,000,000	250,000,000
Loans outstanding	$ 0	$ 55,100,000
Weighted average interest rate	0.00%	4.98%
Remaining borrowing capacity	$ 249,269,000	$ 194,169,000