COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 40,000
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	322,623
2018	320,435
2019	233,395
2020	148,081
2021	114,335
Thereafter	763,226
NJRES
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	237,389
2018	198,769
2019	90,647
2020	21,050
2021	16,427
Thereafter	18,983
NJRES | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	179,517
2018	121,391
2019	55,395
2020	0
2021	0
Thereafter	0
NJRES | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	19,525
2018	28,869
2019	16,195
2020	12,913
2021	8,822
Thereafter	8,580
NJRES | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	38,347
2018	48,509
2019	19,057
2020	8,137
2021	7,605
Thereafter	$ 10,403
NJRES | Minimum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	1 year
NJRES | Maximum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, contract term	10 years
NJNG
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	$ 85,234
2018	121,666
2019	142,748
2020	127,031
2021	97,908
Thereafter	744,243
NJNG | Natural gas purchases
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	45,266
2018	11,250
2019	0
2020	0
2021	0
Thereafter	0
NJNG | Storage demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	14,568
2018	29,187
2019	25,772
2020	14,476
2021	7,802
Thereafter	11,705
NJNG | Pipeline demand fees
Natural Gas Purchases and Future Demand Fees, Next Five Fiscal Years
2017	25,400
2018	81,229
2019	116,976
2020	112,555
2021	90,106
Thereafter	$ 732,538