FINANCIAL INSTRUMENTS - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) counterparty	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of counter parties | counterparty	2
ETG Utility Operations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred incremental costs, CARES Act	$ 9.3
Supplier Concentration Risk | Derivatives Energy Related Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and non current derivatives	$ 5.7
Percentage of current and non current derivatives	12.20%
SJG
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred incremental costs, CARES Act	$ 3.8
SJG | Financing Receivable | Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of loans, net of unamortized discounts	2.8	$ 3.7
Imputed interest of loans	0.4	0.5
Loans for upgrading equipment for energy efficiency	42.8	33.5
Financing Receivable, after Allowance for Credit Loss, Current	5.9	4.6
Loans for upgrading equipment for energy efficiency, noncurrent	$ 36.9	$ 28.9
SJG | Financing Receivable | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Initial term of note (in years)	5 years
SJG | Financing Receivable | Minimum | Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans for upgrading equipment for energy efficiency, term (in years)	2 years
SJG | Financing Receivable | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Initial term of note (in years)	10 years
SJG | Financing Receivable | Maximum | Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans for upgrading equipment for energy efficiency, term (in years)	10 years