REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
SJG Utility Operations
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 454,542	$ 492,365
SJG Utility Operations | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	134,662	136,227
SJG Utility Operations | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	150,041	148,071
SJG Utility Operations | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	32,373	31,096
SJG Utility Operations | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
SJG Utility Operations | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	79,466	80,121
SJG Utility Operations | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	23,572	57,889
SJG Utility Operations | Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	716	2,173
SJG Utility Operations | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	6,582	5,867
SJG Utility Operations | Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	2,136	2,319
SJG Utility Operations | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	594	617
SJG Utility Operations | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	5,078	5,140
SJG Utility Operations | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	10,043	13,914
SJG Utility Operations | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
SJG Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	9,279	8,931
SJI
Regulatory Assets [Line Items]
Total Regulatory Assets	624,187	662,969
SJI | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	149,091	147,102
SJI | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	253,985	252,665
SJI | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	32,373	31,096
SJI | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	39,823	40,626
SJI | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	82,103	82,758
SJI | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	23,642	58,178
SJI | Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	716	2,173
SJI | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	6,582	5,867
SJI | Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	2,136	2,319
SJI | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	594	617
SJI | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	5,078	5,140
SJI | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	10,043	13,914
SJI | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	36	3,349
SJI | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	17,985	17,165
Elizabethtown Gas
Regulatory Assets [Line Items]
Total Regulatory Assets	169,251	169,921
Elizabethtown Gas | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	14,429	10,875
Elizabethtown Gas | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	103,944	104,594
Elizabethtown Gas | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	39,809	40,612
Elizabethtown Gas | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	2,607	2,607
Elizabethtown Gas | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elizabethtown Gas | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	0	3,210
Elizabethtown Gas | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	8,462	8,023
Elkton Gas
Regulatory Assets [Line Items]
Total Regulatory Assets	394	683
Elkton Gas | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	14	14
Elkton Gas | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	30	30
Elkton Gas | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	70	289
Elkton Gas | Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
Elkton Gas | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	36	139
Elkton Gas | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 244	$ 211