FINANCIAL INSTRUMENTS (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) counterparty	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of counter parties \| counterparty	2
Estimated fair value of long-term debt, including current maturities	$ 2,590.0	$ 2,730.0
Carrying amount of long-term debt, including current maturities	2,540.0	2,540.0
Unamortized debt issuance costs	26.0	25.5
Debt discount	5.3	5.3
Level 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Margin accounts with selected counter-parties to support risk management activities	21.7	22.0
Supplier Concentration Risk \| Derivatives Energy Related Assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and non current derivatives	$ 16.9
Percentage of current and non current derivatives	37.80%
SJG
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated fair value of long-term debt, including current maturities	$ 969.0	915.2
Carrying amount of long-term debt, including current maturities	964.6	965.1
Unamortized debt issuance costs	6.7	6.3
SJG \| Financing Receivable \| Level 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying amount of loans, net of unamortized discounts	3.4	3.7
Imputed interest of loans	0.5	0.5
Loans for upgrading equipment for energy efficiency	36.5	33.5
Financing Receivable, after Allowance for Credit Loss, Current	5.1	4.6
Loans for upgrading equipment for energy efficiency, noncurrent	$ 31.4	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
SJG \| Unnamed Counterparty
Accounts, Notes, Loans and Financing Receivable [Line Items]
Margin accounts with selected counter-parties to support risk management activities	$ 5.1	$ 4.0

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