FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Available-for-Sale Securities	$ 32	$ 32
Derivatives - Energy Related Assets	49,718	80,893
Total Assets	49,750	80,925
Liabilities
Derivatives - Energy Related Liabilities	31,269	64,622
Derivatives - Other	11,277	10,030
Total Liabilities	42,546	74,652
SJG
Assets
Derivatives - Energy Related Assets	7,666	5,807
Total Assets	7,666	5,807
Liabilities
Derivatives - Energy Related Liabilities	4,536	1,372
Derivatives - Other	7,279	7,365
Total Liabilities	11,815	8,737
Level 1
Assets
Available-for-Sale Securities	32	32
Derivatives - Energy Related Assets	5,950	33,994
Total Assets	5,982	34,026
Liabilities
Derivatives - Energy Related Liabilities	2,690	16,502
Derivatives - Other	0	0
Total Liabilities	2,690	16,502
Level 1 | SJG
Assets
Derivatives - Energy Related Assets	525	4,767
Total Assets	525	4,767
Liabilities
Derivatives - Energy Related Liabilities	475	6
Derivatives - Other	0	0
Total Liabilities	475	6
Level 2
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	19,296	11,814
Total Assets	19,296	11,814
Liabilities
Derivatives - Energy Related Liabilities	17,873	22,070
Derivatives - Other	11,277	10,030
Total Liabilities	29,150	32,100
Level 2 | SJG
Assets
Derivatives - Energy Related Assets	223	0
Total Assets	223	0
Liabilities
Derivatives - Energy Related Liabilities	2,267	1,252
Derivatives - Other	7,279	7,365
Total Liabilities	9,546	8,617
Level 3
Assets
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	24,472	35,085
Total Assets	24,472	35,085
Liabilities
Derivatives - Energy Related Liabilities	10,706	26,050
Derivatives - Other	0	0
Total Liabilities	10,706	26,050
Level 3 | SJG
Assets
Derivatives - Energy Related Assets	6,918	1,040
Total Assets	6,918	1,040
Liabilities
Derivatives - Energy Related Liabilities	1,794	114
Derivatives - Other	0	0
Total Liabilities	$ 1,794	$ 114