DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	$ 53,609	$ 80,893
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	39,088	74,652
SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	7,998	5,807
Offsetting Derivative Liabilities [Abstract]
Gross amount offset in the balance sheet	9,475	8,737
Commodity Contract
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	53,609	80,893
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	53,609	80,893
Gross amounts not offset in the balance sheet, Financial Instruments	(13,958)	(38,809)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	(2,674)	(3,474)
Net amount	36,977	38,610
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(27,742)	(64,622)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(27,742)	(64,622)
Gross amounts not offset in the balance sheet, Financial Instruments	13,958	38,809
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	(13,784)	(25,813)
Commodity Contract | SJG
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets/liabilities	7,998	5,807
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	7,998	5,807
Gross amounts not offset in the balance sheet, Financial Instruments	(711)	(6)
Gross amounts not offset in the balance sheet, Cash Collateral Posted	(151)	(3,587)
Net amount	7,136	2,214
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(2,178)	(1,372)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(2,178)	(1,372)
Gross amounts not offset in the balance sheet, Financial Instruments	711	6
Gross amounts not offset in the balance sheet, Cash Collateral Posted	0	0
Net amount	(1,467)	(1,366)
Other
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(11,346)	(10,030)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(11,346)	(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	(11,346)	(10,030)
Other | SJG
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets/liabilities	(7,297)	(7,365)
Gross amount offset in the balance sheet	0	0
Net amounts of assets/liabilities in balance sheet	(7,297)	(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	$ (7,297)	$ (7,365)