DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Foreign Currency Contracts [Member] \| Gas Purchases [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 8	$ 3	$ (27)	$ (402)
Amount of Gain or (Loss) Reclassified from OCI into Income (Effective Portion)	(15)	15	27	557
Amount of Gain or (Loss) Recognized on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	0	0	0
NJNG [Member] \| Physical Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Losses) gains on derivative instruments	(236)	0	(14,764)	0
NJNG [Member] \| Financial Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Losses) gains on derivative instruments	4,937	(199)	(5,849)	(24,676)
NJNG [Member] \| Treasury Lock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Losses) gains on derivative instruments	(9,700)	2,905	(18,405)	2,905
Not Designated as Hedging Instrument [Member] \| NJRES [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(38,119)	(3,541)	19,159	92,130
Not Designated as Hedging Instrument [Member] \| NJRES [Member] \| Physical Commodity Contracts [Member] \| Operating Revenues [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	3,997	6,183	24,999	21,130
Not Designated as Hedging Instrument [Member] \| NJRES [Member] \| Physical Commodity Contracts [Member] \| Gas Purchases [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(2,463)	(11,988)	(29,283)	(21,781)
Not Designated as Hedging Instrument [Member] \| NJRES [Member] \| Financial Commodity Contracts [Member] \| Gas Purchases [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(39,653)	2,264	23,443	92,781
Not Designated as Hedging Instrument [Member] \| NJNG [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	$ (4,999)	$ 2,706	$ (39,018)	$ (21,771)

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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=56946850&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=56946850&loc=SL5624171-113959

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+ References

No definition available.

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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=56946850&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=56946850&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=56946850&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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