FINANCIAL INSTRUMENTS - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) counterparty	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of counter parties | counterparty	1
Supplier Concentration Risk | Derivatives Energy Related Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and non current derivatives	$ 6.1
Percentage of current and non current derivatives	11.10%
SJG | Financing Receivable | Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of loans, net of unamortized discounts	$ 2.3	$ 2.5
Loans for upgrading equipment for energy efficiency	49.4	46.4
Current portion of EET loans receivable	6.8	6.4
Loans for upgrading equipment for energy efficiency, noncurrent	$ 42.6	$ 40.0
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