REGULATORY ASSETS & REGULATORY LIABILITIES (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) site regulatory_assets	Dec. 31, 2006 USD ($)	Dec. 31, 2014 USD ($)
Regulatory Assets [Line Items]
Total regulatory assets	$ 323,434		$ 357,160
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) | regulatory_assets	2
Number of sites where SJG or their predecessors previously operated gas manufacturing plants (in sites) | site	12
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory costs, original recovery period of expenditures	7 years
Environmental Remediation Costs: Expended - Net [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 42,032		29,540
Environmental Restoration Costs Liability for Future Expenditures [Member]
Regulatory Assets [Line Items]
Total regulatory assets	123,194		124,308
Deferred Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	42,430		31,584
Deferred Pension and Other Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	79,779		99,040
Regulatory asset, amount of increase		$ 37,100
Deferred Gas Costs - Net [Member]
Regulatory Assets [Line Items]
Total regulatory assets	2,701		32,202
Conservation Incentive Program Receivable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	2,624		0
Societal Benefit Costs Receivable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	0		385
Deferred Interest Rate Contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets	7,631		7,325
Energy Efficiency Tracker [Member]
Regulatory Assets [Line Items]
Total regulatory assets	496		11,247
Pipeline Supplier Service Charges [Member]
Regulatory Assets [Line Items]
Total regulatory assets	3,776		5,441
Pipeline Integrity Cost [Member]
Regulatory Assets [Line Items]
Total regulatory assets	4,596		3,431
AFUDC - Equity Related Deferrals [Member]
Regulatory Assets [Line Items]
Total regulatory assets	11,423		10,781
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total regulatory assets	2,752		$ 1,876
Allowance for Funds Used Under Construction, Equity Related Deferrals, Settlement [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 2,300
Regulatory costs, original recovery period of expenditures	3 years