FINANCIAL INSTRUMENTS - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 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	$ 10.5
Percentage of current and non current derivatives	5.90%
SJG | Financing Receivable | Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of loans, net of unamortized discounts	$ 1.8	$ 2.5
Loans for upgrading equipment for energy efficiency	53.0	46.4
Current portion of EET loans receivable	7.1	6.4
Loans for upgrading equipment for energy efficiency, noncurrent	$ 45.9	$ 40.0
SJG | Financing Receivable | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Initial term of note (in years)	5 years
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
Loans for upgrading equipment for energy efficiency, term (in years)	10 years