REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) site	Dec. 31, 2019 USD ($)
SJG
Regulatory Assets [Line Items]
Regulatory assets	$ 486,070	$ 496,177
Deferred incremental costs, CARES Act	$ 3,800
SJG | Environmental Remediation
Regulatory Assets [Line Items]
Number of sites for environmental cleanup | site	12
Original recovery period of expenditures	7 years
SJG | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Regulatory assets	$ 0	0
SJG | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	25,889	10,678
Increase (decrease) in regulatory assets	10,100
SJG | Judicial Ruling | Pricing dispute, long-term gas supply contract
Regulatory Assets [Line Items]
Amount paid to third party supplier	$ 22,900	22,900
SJG | Unprotected Excess Deferred Income Tax
Regulatory Assets [Line Items]
Period to refund customers	5 years
ETG Utility Operations
Regulatory Assets [Line Items]
Regulatory assets	$ 173,654	169,231
Deferred incremental costs, CARES Act	$ 9,300
ETG Utility Operations | Environmental Remediation
Regulatory Assets [Line Items]
Number of sites for environmental cleanup | site	5
ETG Utility Operations | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Regulatory assets	$ (13,615)	(20,423)
Increase (decrease) in regulatory assets	6,800
ETG Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	$ 18,000	$ 9,004