DEBT - CREDIT FACILITIES (Details) - USD ($)	Dec. 31, 2019	Sep. 30, 2019
NJR | Letter of Credit on Behalf of NJRES
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 4,800,000	$ 4,800,000
NJNG | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	731,000	731,000
Revolving Credit Facility | Committed Credit Facilities Due December 2023 | NJR
Line of Credit Facility [Line Items]
Bank revolving credit facilities	425,000,000	425,000,000
Amount outstanding at end of period	$ 119,800,000	$ 25,450,000
Weighted average interest rate at end of period	2.08%	3.04%
Amount available at end of period	$ 300,450,000	$ 394,800,000
Revolving Credit Facility | Committed Credit Facilities Due December 2023 | NJNG
Line of Credit Facility [Line Items]
Bank revolving credit facilities	250,000,000	250,000,000
Amount outstanding at end of period	$ 49,600,000	$ 0
Weighted average interest rate at end of period	1.83%	0.00%
Amount available at end of period	$ 199,669,000	$ 249,269,000
Revolving Credit Facility | Credit Facilities Due April 2020 | NJR
Line of Credit Facility [Line Items]
Bank revolving credit facilities	150,000,000
Amount outstanding at end of period	$ 85,000,000
Weighted average interest rate at end of period	2.49%
Amount available at end of period	$ 65,000,000