REGULATORY ASSETS & REGULATORY LIABILITIES (Details) (USD $)	12 Months Ended										12 Months Ended
	Dec. 31, 2011 regulatory_assets sites	Dec. 31, 2010	Dec. 31, 2011 Environmental Remediation Costs: Expended - Net [Member]	Dec. 31, 2010 Environmental Remediation Costs: Expended - Net [Member]	Dec. 31, 2011 Environmental Restoration Costs: Liability for Future Expenditures [Member]	Dec. 31, 2010 Environmental Restoration Costs: Liability for Future Expenditures [Member]	Dec. 31, 2011 Income Taxes - Flowthrough Depreciation [Member]	Dec. 31, 2010 Income Taxes - Flowthrough Depreciation [Member]	Dec. 31, 2011 Deferred Asset Retirement Obligation Costs [Member]	Dec. 31, 2010 Deferred Asset Retirement Obligation Costs [Member]	Dec. 31, 2006 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2011 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2010 Deferred Pension and Other Postretirement Benefit Costs [Member]	Dec. 31, 2011 Deferred Gas Costs - Net [Member]	Dec. 31, 2010 Deferred Gas Costs - Net [Member]	Dec. 31, 2011 Conservation Incentive Program Recevable [Member]	Dec. 31, 2010 Conservation Incentive Program Recevable [Member]	Dec. 31, 2011 Societal Benefit Costs Receivable [Member]	Dec. 31, 2010 Societal Benefit Costs Receivable [Member]	Dec. 31, 2011 Premium for Early Retirement of debt [Member]	Dec. 31, 2010 Premium for Early Retirement of debt [Member]	Dec. 31, 2011 Deferred Interest Rate Contracts [Member]	Dec. 31, 2010 Deferred Interest Rate Contracts [Member]	Dec. 31, 2011 Other Regulatory Assets [Member]	Dec. 31, 2010 Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 315,221,000	$ 248,413,000	$ 45,815,000	$ 39,056,000	$ 89,984,000	$ 87,393,000	$ 0	$ 774,000	$ 25,162,000	$ 24,247,000		$ 88,624,000	$ 69,017,000	$ 22,441,000	$ 0	$ 13,580,000	$ 12,291,000	$ 8,618,000	$ 4,216,000	$ 537,000	$ 699,000	$ 8,146,000	$ 3,150,000	$ 12,314,000	$ 7,570,000
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
Amortizatoin period deferred costs	15 years
Unamortized Balance													400,000
Regulatory asset increased											37,100,000
Call premium																				$ 200,000