DEBT - CREDIT FACILITIES (Details) - USD ($)	Jun. 30, 2017	Sep. 30, 2016
NJR | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	$ 15,000,000	$ 14,400,000
NJR | Notes
Line of Credit Facility [Line Items]
Amount outstanding at end of period	$ 263,400,000	$ 121,700,000
Weighted average interest rate at end of period	1.99%	1.43%
Amount available at end of period	$ 146,620,000	$ 288,910,000
NJR | Revolving Credit Facility | Committed Credit Facilities Due September 2020
Line of Credit Facility [Line Items]
Bank revolving credit facilities	425,000,000	425,000,000
NJNG | Commercial Paper
Line of Credit Facility [Line Items]
Amount outstanding at end of period	$ 0	$ 0
Weighted average interest rate at end of period	0.00%	0.00%
Amount available at end of period	$ 249,269,000	$ 249,269,000
NJNG | Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount	731,000	731,000
NJNG | Revolving Credit Facility | Committed Credit Facilities Due May 2019
Line of Credit Facility [Line Items]
Bank revolving credit facilities	$ 250,000,000	$ 250,000,000