DEBT - CREDIT FACILITIES AND SHORT-TERM DEBT (Details) - USD ($)	6 Months Ended
	Mar. 31, 2022	Sep. 30, 2021
NJR | Letter of Credit on Behalf of NJRES
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 13,500,000	$ 10,600,000
NJNG | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 731,000	731,000
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%
Term Loan | Committed Credit Facilities Due February 2023 | NJR
Line of Credit Facility [Line Items]
Bank revolving credit facilities	$ 150,000,000	0
Amount outstanding at end of period	$ 150,000,000	$ 0
Weighted average interest rate at end of period	1.04%	0.00%
Amount available at end of period	$ 0	$ 0
Revolving Credit Facility | Committed Credit Facilities Due September 2026 | NJR
Line of Credit Facility [Line Items]
Bank revolving credit facilities	500,000,000	500,000,000
Amount outstanding at end of period	$ 152,210,000	$ 219,100,000
Weighted average interest rate at end of period	2.02%	1.05%
Amount available at end of period	$ 334,325,000	$ 270,312,000
Revolving Credit Facility | Committed Credit Facilities Due September 2026 | NJNG
Line of Credit Facility [Line Items]
Bank revolving credit facilities	250,000,000	250,000,000
Amount outstanding at end of period	$ 0	$ 158,200,000
Weighted average interest rate at end of period	0.00%	0.17%
Amount available at end of period	$ 249,269,000	$ 91,069,000