FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	$ 36	$ 36
Assets
Derivatives - Energy Related Assets	35,428	48,127
Total Assets	35,464	48,163
Liabilities
Derivatives - Energy Related Liabilities	27,466	52,963
Derivatives - Other	5,464	10,370
Total Liabilities	32,930	63,333
SJG
Assets
Derivatives - Energy Related Assets	7,953	7,332
Total Assets	7,953	7,332
Liabilities
Derivatives - Energy Related Liabilities	2,872	9,440
Derivatives - Other	5,042	7,028
Total Liabilities	7,914	16,468
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	36	36
Assets
Derivatives - Energy Related Assets	1,817	5,155
Total Assets	1,853	5,191
Liabilities
Derivatives - Energy Related Liabilities	3,991	10,687
Derivatives - Other	0	0
Total Liabilities	3,991	10,687
Level 1 | SJG
Assets
Derivatives - Energy Related Assets	405	208
Total Assets	405	208
Liabilities
Derivatives - Energy Related Liabilities	287	1,750
Derivatives - Other	0	0
Total Liabilities	287	1,750
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0	0
Assets
Derivatives - Energy Related Assets	10,952	21,869
Total Assets	10,952	21,869
Liabilities
Derivatives - Energy Related Liabilities	11,120	24,283
Derivatives - Other	5,464	10,370
Total Liabilities	16,584	34,653
Level 2 | SJG
Assets
Derivatives - Energy Related Assets	0	230
Total Assets	0	230
Liabilities
Derivatives - Energy Related Liabilities	1,653	2,848
Derivatives - Other	5,042	7,028
Total Liabilities	6,695	9,876
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale	0	0
Assets
Derivatives - Energy Related Assets	22,659	21,103
Total Assets	22,659	21,103
Liabilities
Derivatives - Energy Related Liabilities	12,355	17,993
Derivatives - Other	0	0
Total Liabilities	12,355	17,993
Level 3 | SJG
Assets
Derivatives - Energy Related Assets	7,548	6,894
Total Assets	7,548	6,894
Liabilities
Derivatives - Energy Related Liabilities	932	4,842
Derivatives - Other	0	0
Total Liabilities	$ 932	$ 4,842