DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Energy-related commodity contracts
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 54,625	$ 48,374
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	54,625	48,374
Gross amounts not offset in the balance sheet, Financial Instruments	(26,491)	(24,027)
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	(5,163)	0
Net amount	22,971	24,347
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(33,655)	(31,953)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(33,655)	(31,953)
Gross amounts not offset in the balance sheet, Financial Instruments	26,491	24,027
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	2,176
Net amount	(7,164)	(5,750)
Energy-related commodity contracts \| SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	4,382	4,140
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	4,382	4,140
Gross amounts not offset in the balance sheet, Financial Instruments	(613)	(716)
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	0
Net amount	3,769	3,424
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(627)	(3,058)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(627)	(3,058)
Gross amounts not offset in the balance sheet, Financial Instruments	613	716
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	2,176
Net amount	(14)	(166)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(7,441)	(9,938)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(7,441)	(9,938)
Gross amounts not offset in the balance sheet, Financial Instruments	0	0
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	0
Net amount	(7,441)	(9,938)
Other \| SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(7,441)	(9,938)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(7,441)	(9,938)
Gross amounts not offset in the balance sheet, Financial Instruments	0	0
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	0
Net amount	$ (7,441)	$ (9,938)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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