DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 49,718	$ 80,893
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	42,546	74,652
SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	7,666	5,807
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	11,815	8,737
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	49,718	80,893
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	49,718	80,893
Gross amounts not offset in the balance sheet, Financial Instruments	(18,251)	(38,809)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	(3,474)
Net amount	31,467	38,610
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(31,269)	(64,622)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(31,269)	(64,622)
Gross amounts not offset in the balance sheet, Financial Instruments	18,251	38,809
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	(13,018)	(25,813)
Commodity Contract | SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	7,666	5,807
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	7,666	5,807
Gross amounts not offset in the balance sheet, Financial Instruments	(475)	(6)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	(3,587)
Net amount	7,191	2,214
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(4,536)	(1,372)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(4,536)	(1,372)
Gross amounts not offset in the balance sheet, Financial Instruments	475	6
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	(4,061)	(1,366)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(11,277)	(10,030)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(11,277)	(10,030)
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	(11,277)	(10,030)
Other | SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(7,279)	(7,365)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(7,279)	(7,365)
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	$ (7,279)	$ (7,365)