DERIVATIVE INSTRUMENTS, INCOME STATEMENT RELATED DISCLOSURES (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Foreign currency contracts \| Gas purchases \| Derivatives in cash flow hedging relationships:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ (27)	$ (402)
Amount of Gain or (Loss) Reclassified from OCI into Income (Effective Portion)	27	557
Amount of Gain or (Loss) Recognized on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	0
NJRES \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	32,942	107,212	$ (251,696)
NJRES \| Physical commodity contracts \| Operating revenues \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	33,034	32,568	(48,977)
NJRES \| Physical commodity contracts \| Gas purchases \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(45,637)	(34,438)	(83,847)
NJRES \| Financial commodity contracts \| Gas purchases \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	45,579	109,082	(118,872)
NJRES \| Foreign currency contracts \| Gas purchases \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(34)	0	0
NJNG \| Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(42,585)	(37,656)	10,149
NJNG \| Physical commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments gain (loss) recognized in regulatory assets and liabilities, net	(15,756)	0	0
NJNG \| Financial commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments gain (loss) recognized in regulatory assets and liabilities, net	(7,984)	(33,428)	10,149
NJNG \| Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments gain (loss) recognized in regulatory assets and liabilities, net	$ (18,845)	$ (4,228)	$ 0

Net gain or loss on derivative instruments at NJNG that is deferred and recognized in regulatory assets and liabilities

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

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+ References

No definition available.

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624177-113959

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- Definition

The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1),(2)
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

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