DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 42,857	$ 61,190
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	36,783	39,263
SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	2,076	5,479
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	7,371	8,056
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	42,857	61,190
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	42,857	61,190
Gross amounts not offset in the balance sheet, Financial Instruments	(23,142)	(21,045)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	(7,252)
Net amount	19,715	32,893
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(27,105)	(31,390)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(27,105)	(31,390)
Gross amounts not offset in the balance sheet, Financial Instruments	23,142	21,045
Gross amounts not offset in the balance sheet, Cash Collateral Posted	479	0
Net amount	(3,484)	(10,345)
Commodity Contract \| SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	2,076	5,479
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	2,076	5,479
Gross amounts not offset in the balance sheet, Financial Instruments	(303)	(347)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	688
Net amount	1,773	5,820
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(789)	(2,189)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(789)	(2,189)
Gross amounts not offset in the balance sheet, Financial Instruments	303	347
Gross amounts not offset in the balance sheet, Cash Collateral Posted	479	0
Net amount	(7)	(1,842)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(9,678)	(7,873)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(9,678)	(7,873)
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	(9,678)	(7,873)
Other \| SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(6,582)	(5,867)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(6,582)	(5,867)
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	$ (6,582)	$ (5,867)

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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