DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 36,654	$ 48,127
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	24,359	63,333
SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	404	7,332
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	7,118	16,468
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	36,654	48,127
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	36,654	48,127
Gross amounts not offset in the balance sheet, Financial Instruments	(10,086)	(24,849)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	26,568	23,278
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(16,639)	(52,963)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(16,639)	(52,963)
Gross amounts not offset in the balance sheet, Financial Instruments	10,086	24,849
Gross amounts not offset in the balance sheet, Cash Collateral Posted	755	8,832
Net amount	(5,798)	(19,282)
Commodity Contract | SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	404	7,332
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	404	7,332
Gross amounts not offset in the balance sheet, Financial Instruments	(342)	(208)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	62	7,124
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(1,112)	(9,440)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(1,112)	(9,440)
Gross amounts not offset in the balance sheet, Financial Instruments	342	208
Gross amounts not offset in the balance sheet, Cash Collateral Posted	755	1,543
Net amount	(15)	(7,689)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(7,720)	(10,370)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(7,720)	(10,370)
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	(7,720)	(10,370)
Other | SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(6,006)	(7,028)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(6,006)	(7,028)
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	$ (6,006)	$ (7,028)