DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Energy-related commodity contracts
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 43,411	$ 60,135
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	43,411	60,135
Gross amounts not offset in the balance sheet, Financial Instruments	(23,219)	(32,185)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	20,192	27,950
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(31,268)	(50,171)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(31,268)	(50,171)
Gross amounts not offset in the balance sheet, Financial Instruments	23,219	32,185
Gross amounts not offset in the balance sheet, Cash Collateral Posted	4,297	12,878
Net amount	(3,752)	(5,108)
Energy-related commodity contracts \| SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	4,842	16,909
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	4,842	16,909
Gross amounts not offset in the balance sheet, Financial Instruments	(369)	(11,860)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	4,473	5,049
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(2,651)	(14,766)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(2,651)	(14,766)
Gross amounts not offset in the balance sheet, Financial Instruments	369	11,860
Gross amounts not offset in the balance sheet, Cash Collateral Posted	2,085	2,706
Net amount	(197)	(200)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(20,592)	(12,660)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(20,592)	(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	0	0
Net amount	(20,592)	(12,660)
Other \| SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(11,407)	(7,856)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(11,407)	(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	0	0
Net amount	$ (11,407)	$ (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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