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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Gas in Storage (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 MMcf	Sep. 30, 2012 MMcf
Public Utilities, Inventory [Line Items]
Gas in storage	$ 315,945	$ 265,193
Gas in storage (BCF)	77,400	67,700
NJNG [Member]
Public Utilities, Inventory [Line Items]
Gas in storage	123,624	145,379
Gas in storage (BCF)	19,800	22,200
NJRES [Member]
Public Utilities, Inventory [Line Items]
Gas in storage	$ 192,321	$ 119,814
Gas in storage (BCF)	57,600	45,500

X

- Definition

Gas Stored Underground Nonmonetary Number of Billion Cubic Feet of Gas in Storage as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	njr_GasStoredUndergroundBcf
Namespace Prefix:	njr_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 6

-Subparagraph a

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_PublicUtilitiesInventoryLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration