REGULATORY ASSETS AND LIABILITIES - Assets (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) site asset	Dec. 31, 2014 USD ($)	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 Restoration Costs
Regulatory Assets [Line Items]
Approved recoverable period	7 years
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
AFUDC Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets		$ 1,000
Approved recoverable period		3 years