COMMITMENTS AND CONTINGENCIES (Details) (USD $)	6 Months Ended
Jun. 30, 2012 unions sites	Dec. 31, 2011
Loss Contingencies [Line Items]
Letter of credit provided	$ 210,000,000
Number of sites for environmental cleanup	12
Monthly gas supply related demand charges and reservation fees	3,600,000
Estimated litigation liability	400,000	200,000
Percentage of personnel represented in collective bargaining agreements	63.00%
Number of unions	2
Standby Letter of Credit [Member]
Loss Contingencies [Line Items]
Letter of credit provided	$ 25,200,000

Cost of Services, Oil and Gas, Monthly

No definition available.

Name:	sjg_CostOfServicesOilAndGasMonthly
Namespace Prefix:	sjg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of environmental cleanup sites for which costs have been incurred and recorded.

+ References

No definition available.

+ Details

Name:	sjg_NumberOfSitesForEnvironmentalCleanup
Namespace Prefix:	sjg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of unions that represent employees covered by collective bargaining agreements.

+ References

No definition available.

+ Details

Name:	sjg_NumberOfUnionsCollectiveBargainingAgreements
Namespace Prefix:	sjg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of personnel represented by collective bargaining agreements.

+ References

No definition available.

+ Details

Name:	sjg_PercentageOfPersonnelRepresentedCollectiveBargainingAgreements
Namespace Prefix:	sjg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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 210.5-02.19(b),22(b))

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349

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 210.5-02.25)

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

+ Details

Name:	us-gaap_LitigationReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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