REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative and Regulatory Liabilities (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) site	Dec. 31, 2019 USD ($)
SJG
Regulatory Assets [Line Items]
Regulatory assets	$ 518,184	$ 496,177
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 | Deferred Revenues
Regulatory Assets [Line Items]
Increase (decrease) in regulatory assets	(21,300)
SJG | Deferred ARO Costs
Regulatory Assets [Line Items]
Regulatory assets	37,821	36,515
SJG | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	20,918	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
SJI
Regulatory Assets [Line Items]
Regulatory assets	$ 699,426	665,932
SJI | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Regulatory assets	(13,615)	(20,423)
SJI | Deferred Revenues
Regulatory Assets [Line Items]
Increase (decrease) in regulatory assets	(26,900)
SJI | Deferred ARO Costs
Regulatory Assets [Line Items]
Regulatory assets	61,332	54,623
Increase (decrease) in regulatory assets	6,700
SJI | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	28,955	19,682
ETG Utility Operations
Regulatory Assets [Line Items]
Regulatory assets	$ 181,086	169,231
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 | Deferred ARO Costs
Regulatory Assets [Line Items]
Regulatory assets	23,511	18,108
ETG Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	$ 8,037	$ 9,004