REGULATORY ASSETS & REGULATORY LIABILITIES - ASSETS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) site asset	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 410,746	$ 323,434
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) | asset	2
Number of sites where SJG or their predecessors previously operated gas manufacturing plants (in sites) | site	12
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 71,997	42,032
Environmental Restoration Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	153,047	123,194
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	43,014	42,430
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	85,693	79,779
Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	2,701
Conservation Incentive Program Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	27,567	2,624
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	7,365	7,631
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	219	496
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	2,122	3,776
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	4,810	4,596
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	12,434	11,423
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 2,478	$ 2,752
Environmental restoration costs
Regulatory Assets [Line Items]
Regulatory costs, original recovery period of expenditures	7 years
Allowance for Funds Used Under Construction, Equity Related Deferrals, settlement
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 1,000
Regulatory costs, original recovery period of expenditures	3 years