REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 477,457	$ 410,746
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	93,724	71,997
Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	176,805	153,047
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	42,283	43,014
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	85,693	85,693
Conservation Incentive Program Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	32,429	27,567
Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	906	0
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	7,280	7,365
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	1,994	219
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	862	2,122
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	5,160	4,810
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	12,331	12,434
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	2,745	2,478
South Jersey Gas Company | Deferred Gas Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 15,245	$ 0