REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
ETG Utility Operations
Regulatory Assets [Line Items]
Deferral of costs related to expected credit losses from uncollectibles	$ 7,800	$ 5,800
SJG
Regulatory Assets [Line Items]
Regulatory assets	473,402	495,084
Deferral of costs related to expected credit losses from uncollectibles	6,800	$ 4,700
SJG | Environmental Remediation
Regulatory Assets [Line Items]
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	27,153	26,056
SJG | Judicial Ruling | Pricing dispute, long-term gas supply contract
Regulatory Assets [Line Items]
Amount paid to third party supplier	22,900	22,900
ETG Utility Operations
Regulatory Assets [Line Items]
Regulatory assets	177,308	178,908
ETG Utility Operations | Environmental Remediation Costs: Insurance Recovery Receivables
Regulatory Assets [Line Items]
Regulatory assets	(6,807)	(6,807)
ETG Utility Operations | Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	$ 10,971	$ 10,359