DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Energy-related commodity contracts
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 46,948	$ 60,135
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	46,948	60,135
Gross amounts not offset in the balance sheet, Financial Instruments	(24,937)	(32,185)
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	(3,706)	0
Net amount	18,305	27,950
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(29,457)	(50,171)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(29,457)	(50,171)
Gross amounts not offset in the balance sheet, Financial Instruments	24,937	32,185
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	12,878
Net amount	(4,520)	(5,108)
Energy-related commodity contracts \| SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	6,485	16,909
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	6,485	16,909
Gross amounts not offset in the balance sheet, Financial Instruments	(338)	(11,860)
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	0
Net amount	6,147	5,049
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(817)	(14,766)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(817)	(14,766)
Gross amounts not offset in the balance sheet, Financial Instruments	338	11,860
Gross amounts not offset in the balance sheet, Cash Collateral Posted/(Received)	0	2,706
Net amount	(479)	(200)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(19,715)	(12,660)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(19,715)	(12,660)
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	(19,715)	(12,660)
Other \| SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(10,872)	(7,856)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(10,872)	(7,856)
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	$ (10,872)	$ (7,856)

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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