FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-Sale Securities	$ 41	$ 41
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-Sale Securities	41	41
Assets
Derivatives - Energy Related Assets	42,857	61,190
Total Assets	42,898	61,231
Liabilities
Derivatives - Energy Related Liabilities	27,105	31,390
Derivatives - Other	9,678	7,873
Total Liabilities	36,783	39,263
Fair Value, Measurements, Recurring | SJG
Assets
Derivatives - Energy Related Assets	2,076	5,479
Total Assets	2,076	5,479
Liabilities
Derivatives - Energy Related Liabilities	789	2,189
Derivatives - Other	6,582	5,867
Total Liabilities	7,371	8,056
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-Sale Securities	41	41
Assets
Derivatives - Energy Related Assets	4,867	9,955
Total Assets	4,908	9,996
Liabilities
Derivatives - Energy Related Liabilities	2,227	7,291
Derivatives - Other	0	0
Total Liabilities	2,227	7,291
Fair Value, Measurements, Recurring | Level 1 | SJG
Assets
Derivatives - Energy Related Assets	303	348
Total Assets	303	348
Liabilities
Derivatives - Energy Related Liabilities	783	1,035
Derivatives - Other	0	0
Total Liabilities	783	1,035
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-Sale Securities	0	0
Assets
Derivatives - Energy Related Assets	16,523	23,429
Total Assets	16,523	23,429
Liabilities
Derivatives - Energy Related Liabilities	11,688	12,354
Derivatives - Other	9,678	7,873
Total Liabilities	21,366	20,227
Fair Value, Measurements, Recurring | Level 2 | SJG
Assets
Derivatives - Energy Related Assets	64	126
Total Assets	64	126
Liabilities
Derivatives - Energy Related Liabilities	3	1,077
Derivatives - Other	6,582	5,867
Total Liabilities	6,585	6,944
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-Sale Securities	0	0
Assets
Derivatives - Energy Related Assets	21,467	27,806
Total Assets	21,467	27,806
Liabilities
Derivatives - Energy Related Liabilities	13,190	11,745
Derivatives - Other	0	0
Total Liabilities	13,190	11,745
Fair Value, Measurements, Recurring | Level 3 | SJG
Assets
Derivatives - Energy Related Assets	1,709	5,005
Total Assets	1,709	5,005
Liabilities
Derivatives - Energy Related Liabilities	3	77
Derivatives - Other	0	0
Total Liabilities	$ 3	$ 77