DEBT - CREDIT FACILITIES AND SHORT-TERM DEBT (Details) - USD ($)	9 Months Ended
	Jun. 30, 2024	Sep. 30, 2023
NJR | Letter of Credit on Behalf of NJRES
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 9,300,000	$ 5,700,000
NJNG | Letter of Credit
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 September 2027 | NJR
Line of Credit Facility [Line Items]
Total borrowing capacity	$ 650,000,000	650,000,000
Loans outstanding	$ 254,800,000	$ 217,300,000
Weighted average interest rate	6.54%	6.53%
Remaining borrowing capacity	$ 385,869,000	$ 426,967,000
Revolving Credit Facility | Committed Credit Facilities Due September 2027 | NJNG
Line of Credit Facility [Line Items]
Total borrowing capacity	250,000,000	250,000,000
Loans outstanding	$ 0	$ 34,800,000
Weighted average interest rate	0.00%	5.48%
Remaining borrowing capacity	$ 249,269,000	$ 214,469,000