FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets
Available-for-Sale Securities	$ 32	$ 32
Derivatives - Energy Related Assets	53,609	80,893
Total Assets	53,641	80,925
Liabilities
Derivatives - Energy Related Liabilities	27,742	64,622
Derivatives - Other	11,346	10,030
Total Liabilities	39,088	74,652
SJG
Assets
Derivatives - Energy Related Assets	7,998	5,807
Total Assets	7,998	5,807
Liabilities
Derivatives - Energy Related Liabilities	2,178	1,372
Derivatives - Other	7,297	7,365
Total Liabilities	9,475	8,737
Level 1
Assets
Available-for-Sale Securities	32	32
Derivatives - Energy Related Assets	6,499	33,994
Total Assets	6,531	34,026
Liabilities
Derivatives - Energy Related Liabilities	4,256	16,502
Derivatives - Other	0	0
Total Liabilities	4,256	16,502
Level 1 | SJG
Assets
Derivatives - Energy Related Assets	711	4,767
Total Assets	711	4,767
Liabilities
Derivatives - Energy Related Liabilities	862	6
Derivatives - Other	0	0
Total Liabilities	862	6
Level 2
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	18,985	11,814
Total Assets	18,985	11,814
Liabilities
Derivatives - Energy Related Liabilities	12,762	22,070
Derivatives - Other	11,346	10,030
Total Liabilities	24,108	32,100
Level 2 | SJG
Assets
Derivatives - Energy Related Assets	46	0
Total Assets	46	0
Liabilities
Derivatives - Energy Related Liabilities	1,008	1,252
Derivatives - Other	7,297	7,365
Total Liabilities	8,305	8,617
Level 3
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	28,125	35,085
Total Assets	28,125	35,085
Liabilities
Derivatives - Energy Related Liabilities	10,724	26,050
Derivatives - Other	0	0
Total Liabilities	10,724	26,050
Level 3 | SJG
Assets
Derivatives - Energy Related Assets	7,241	1,040
Total Assets	7,241	1,040
Liabilities
Derivatives - Energy Related Liabilities	308	114
Derivatives - Other	0	0
Total Liabilities	$ 308	$ 114