REGULATORY ASSETS & REGULATORY LIABILITIES (Details) (USD $)	12 Months Ended										12 Months Ended
	Dec. 31, 2012 regulatory_assets sites	Dec. 31, 2011	Dec. 31, 2012 Deferred Revenues - Net [Member]	Dec. 31, 2011 Deferred Revenues - Net [Member]	Dec. 31, 2012 Environmental Remediation Costs: Expended - Net [Member]	Dec. 31, 2011 Environmental Remediation Costs: Expended - Net [Member]	Dec. 31, 2012 Environmental Restoration Costs: Liability for Future Expenditures [Member]	Dec. 31, 2011 Environmental Restoration Costs: Liability for Future Expenditures [Member]	Dec. 31, 2012 Deferred Asset Retirement Obligation Costs [Member]	Dec. 31, 2011 Deferred Asset Retirement Obligation Costs [Member]	Dec. 31, 2006 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2012 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2011 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2012 Deferred Gas Costs - Net [Member]	Dec. 31, 2011 Deferred Gas Costs - Net [Member]	Dec. 31, 2012 Conservation Incentive Program Receivable [Member]	Dec. 31, 2011 Conservation Incentive Program Receivable [Member]	Dec. 31, 2012 Societal Benefit Costs Receivable [Member]	Dec. 31, 2011 Societal Benefit Costs Receivable [Member]	Dec. 31, 2012 Premium for Early Retirement of debt [Member]	Dec. 31, 2011 Premium for Early Retirement of debt [Member]	Dec. 31, 2012 Deferred Interest Rate Contracts [Member]	Dec. 31, 2011 Deferred Interest Rate Contracts [Member]	Dec. 31, 2012 Energy Efficiency Tracker [Member]	Dec. 31, 2011 Energy Efficiency Tracker [Member]	Dec. 31, 2012 Pipeline Supplier Service Charges [Member]	Dec. 31, 2011 Pipeline Supplier Service Charges [Member]	Dec. 31, 2012 Other Regulatory Assets [Member]	Dec. 31, 2011 Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 352,656,000	$ 315,221,000			$ 37,892,000	$ 45,815,000	$ 107,410,000	$ 89,984,000	$ 30,199,000	$ 25,162,000		$ 95,897,000	$ 88,624,000	$ 0	$ 22,441,000	$ 31,686,000	$ 13,580,000	$ 12,801,000	$ 8,618,000	$ 1,075,000	$ 537,000	$ 7,761,000	$ 8,146,000	$ 12,306,000	$ 8,464,000	$ 8,771,000	$ 0	$ 6,858,000	$ 3,850,000
Number regulatory assets associated with environmental costs related to the cleanup of 12 sites where SJG or their predecessors previously operated gas manufacturing plants (in regulatory assets)	2
Number of sites where SJG or their predecessors previously operated gas manufacturing plants (in sites)	12
Regulatory liabilities	56,517,000	48,311,000	10,924,000	0
Amortization period deferred costs	15 years
Regulatory asset increased											$ 37,100,000