FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Available-for-Sale Securities	$ 36	$ 36
Derivatives - Energy Related Assets	36,654	48,127
Total Assets	36,690	48,163
Liabilities
Derivatives - Energy Related Liabilities	16,639	52,963
Derivatives - Other	7,720	10,370
Total Liabilities	24,359	63,333
SJG
Assets
Derivatives - Energy Related Assets	404	7,332
Total Assets	404	7,332
Liabilities
Derivatives - Energy Related Liabilities	1,112	9,440
Derivatives - Other	6,006	7,028
Total Liabilities	7,118	16,468
Level 1
Assets
Available-for-Sale Securities	36	36
Derivatives - Energy Related Assets	935	5,155
Total Assets	971	5,191
Liabilities
Derivatives - Energy Related Liabilities	3,947	10,687
Derivatives - Other	0	0
Total Liabilities	3,947	10,687
Level 1 | SJG
Assets
Derivatives - Energy Related Assets	342	208
Total Assets	342	208
Liabilities
Derivatives - Energy Related Liabilities	1,097	1,750
Derivatives - Other	0	0
Total Liabilities	1,097	1,750
Level 2
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	14,121	21,869
Total Assets	14,121	21,869
Liabilities
Derivatives - Energy Related Liabilities	7,680	24,283
Derivatives - Other	7,720	10,370
Total Liabilities	15,400	34,653
Level 2 | SJG
Assets
Derivatives - Energy Related Assets	62	230
Total Assets	62	230
Liabilities
Derivatives - Energy Related Liabilities	9	2,848
Derivatives - Other	6,006	7,028
Total Liabilities	6,015	9,876
Level 3
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	21,598	21,103
Total Assets	21,598	21,103
Liabilities
Derivatives - Energy Related Liabilities	5,012	17,993
Derivatives - Other	0	0
Total Liabilities	5,012	17,993
Level 3 | SJG
Assets
Derivatives - Energy Related Assets	0	6,894
Total Assets	0	6,894
Liabilities
Derivatives - Energy Related Liabilities	6	4,842
Derivatives - Other	0	0
Total Liabilities	$ 6	$ 4,842