REGULATORY ASSETS & REGULATORY LIABILITIES - ASSETS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) site asset	Dec. 31, 2016 USD ($)
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 469,224	$ 410,746
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	100,327	71,997
Environmental Restoration Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	171,696	153,047
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	42,368	43,014
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	78,211	85,693
Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	16,838	0
Conservation Incentive Program Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	26,652	27,567
Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	2,484	0
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	7,028	7,365
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	2,094	219
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	708	2,122
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	5,280	4,810
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	12,785	12,434
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 2,753	$ 2,478
South Jersey Gas Company
Regulatory Assets [Line Items]
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
South Jersey Gas Company | Environmental restoration costs
Regulatory Assets [Line Items]
Regulatory costs, original recovery period of expenditures	7 years
South Jersey Gas Company | 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