REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
SJG
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 518,184	$ 496,177
SJG | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	158,947	156,279
SJG | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	132,938	131,262
SJG | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
SJG | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	37,821	36,515
SJG | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
SJG | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	72,010	72,010
SJG | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	28,171	49,469
SJG | CIP Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	24,244
SJG | SBC Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	1,670	1,478
SJG | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	11,375	7,856
SJG | EET
Regulatory Assets [Line Items]
Total Regulatory Assets	12,557	12,877
SJG | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	503	525
SJG | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	6,080	6,516
SJG | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	10,950	10,712
SJG | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
SJG | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	20,918	10,678
SJI
Regulatory Assets [Line Items]
Total Regulatory Assets	699,426	665,932
SJI | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	169,511	173,234
SJI | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	238,922	232,345
SJI | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Total Regulatory Assets	(13,615)	(20,423)
SJI | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	61,332	54,623
SJI | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	36,566	37,378
SJI | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	73,835	73,835
SJI | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	28,171	55,063
SJI | CIP Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	24,244
SJI | SBC Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	1,670	1,478
SJI | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	11,375	7,856
SJI | EET
Regulatory Assets [Line Items]
Total Regulatory Assets	12,557	12,877
SJI | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	503	525
SJI | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	6,080	6,516
SJI | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	10,950	10,712
SJI | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	8,370	231
SJI | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	28,955	19,682
ETG Utility Operations
Regulatory Assets [Line Items]
Total Regulatory Assets	181,086	169,231
ETG Utility Operations | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	10,564	16,955
ETG Utility Operations | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	105,984	101,083
ETG Utility Operations | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Total Regulatory Assets	(13,615)	(20,423)
ETG Utility Operations | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	23,511	18,108
ETG Utility Operations | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	36,566	37,378
ETG Utility Operations | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	1,825	1,825
ETG Utility Operations | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	5,301
ETG Utility Operations | CIP Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0
ETG Utility Operations | SBC Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | EET
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ETG Utility Operations | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	8,214	0
ETG Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	8,037	9,004
ELK Utility Operations
Regulatory Assets [Line Items]
Total Regulatory Assets	156	524
ELK Utility Operations | Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Deferred ARO Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Deferred Pension Costs - Unrecognized Prior Service Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	0	293
ELK Utility Operations | CIP Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0
ELK Utility Operations | SBC Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | EET
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	0	0
ELK Utility Operations | Weather Normalization
Regulatory Assets [Line Items]
Total Regulatory Assets	156	231
ELK Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 0	$ 0