DERIVATIVE INSTRUMENTS - OFFSETTING OF ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
NJRES
Derivative assets:
Amounts Presented in Balance Sheets	$ 30,520	$ 34,076
Offsetting Derivative Instruments	(16,863)	(20,783)
Financial Collateral Received/Pledged	(5,043)	(6,904)
Net Amounts	8,614	6,389
Derivative liabilities:
Amounts Presented in Balance Sheets	20,661	58,740
Offsetting Derivative Instruments	(16,863)	(20,783)
Financial Collateral Received/Pledged	0	(26,691)
Net Amounts	3,798	11,266
NJRES | Physical commodity contracts
Derivative assets:
Amounts Presented in Balance Sheets	10,122	9,981
Offsetting Derivative Instruments	(1,496)	(2,837)
Financial Collateral Received/Pledged	(200)	(755)
Net Amounts	8,426	6,389
Derivative liabilities:
Amounts Presented in Balance Sheets	5,294	12,872
Offsetting Derivative Instruments	(1,496)	(2,837)
Financial Collateral Received/Pledged	0	1,200
Net Amounts	3,798	11,235
NJRES | Financial commodity contracts
Derivative assets:
Amounts Presented in Balance Sheets	20,382	24,094
Offsetting Derivative Instruments	(15,367)	(17,945)
Financial Collateral Received/Pledged	(4,843)	(6,149)
Net Amounts	172	0
Derivative liabilities:
Amounts Presented in Balance Sheets	15,367	45,836
Offsetting Derivative Instruments	(15,367)	(17,945)
Financial Collateral Received/Pledged	0	(27,891)
Net Amounts	0	0
NJRES | Foreign currency contracts
Derivative assets:
Amounts Presented in Balance Sheets	16	1
Offsetting Derivative Instruments	0	(1)
Financial Collateral Received/Pledged	0	0
Net Amounts	16	0
Derivative liabilities:
Amounts Presented in Balance Sheets		32
Offsetting Derivative Instruments		(1)
Financial Collateral Received/Pledged		0
Net Amounts		31
NJNG
Derivative assets:
Amounts Presented in Balance Sheets	113	1,115
Offsetting Derivative Instruments	(24)	(911)
Financial Collateral Received/Pledged	0	0
Net Amounts	89	204
Derivative liabilities:
Amounts Presented in Balance Sheets	9,534	27,592
Offsetting Derivative Instruments	(24)	(911)
Financial Collateral Received/Pledged	(904)	(2,485)
Net Amounts	8,606	24,196
NJNG | Physical commodity contracts
Derivative assets:
Amounts Presented in Balance Sheets	92	235
Offsetting Derivative Instruments	(3)	(31)
Financial Collateral Received/Pledged	0	0
Net Amounts	89	204
Derivative liabilities:
Amounts Presented in Balance Sheets	15	1,154
Offsetting Derivative Instruments	(3)	(31)
Financial Collateral Received/Pledged	0	0
Net Amounts	12	1,123
NJNG | Financial commodity contracts
Derivative assets:
Amounts Presented in Balance Sheets	21	880
Offsetting Derivative Instruments	(21)	(880)
Financial Collateral Received/Pledged	0	0
Net Amounts	0	0
Derivative liabilities:
Amounts Presented in Balance Sheets	925	3,365
Offsetting Derivative Instruments	(21)	(880)
Financial Collateral Received/Pledged	(904)	(2,485)
Net Amounts	0	0
NJNG | Interest rate contracts
Derivative liabilities:
Amounts Presented in Balance Sheets	8,594	23,073
Offsetting Derivative Instruments	0	0
Financial Collateral Received/Pledged	0	0
Net Amounts	$ 8,594	$ 23,073