FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	$ 32	$ 32
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	32	32
Derivatives - Energy Related Assets	54,625	48,374
Total Assets	54,657	48,406
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	33,655	31,953
Derivatives - Other	7,441	9,938
Total Liabilities	41,096	41,891
Fair Value, Measurements, Recurring | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	4,382	4,140
Total Assets	4,382	4,140
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	627	3,058
Derivatives - Other	7,441	9,938
Total Liabilities	8,068	12,996
Fair Value, Measurements, Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	32	32
Derivatives - Energy Related Assets	11,509	11,447
Total Assets	11,541	11,479
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	3,582	8,605
Derivatives - Other	0	0
Total Liabilities	3,582	8,605
Fair Value, Measurements, Recurring | Level 1 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	974	715
Total Assets	974	715
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	613	2,891
Derivatives - Other	0	0
Total Liabilities	613	2,891
Fair Value, Measurements, Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	30,544	23,527
Total Assets	30,544	23,527
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	28,921	20,954
Derivatives - Other	7,441	9,938
Total Liabilities	36,362	30,892
Fair Value, Measurements, Recurring | Level 2 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	6	32
Total Assets	6	32
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	3	159
Derivatives - Other	7,441	9,938
Total Liabilities	7,444	10,097
Fair Value, Measurements, Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	12,572	13,400
Total Assets	12,572	13,400
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	1,152	2,394
Derivatives - Other	0	0
Total Liabilities	1,152	2,394
Fair Value, Measurements, Recurring | Level 3 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	3,402	3,393
Total Assets	3,402	3,393
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	11	8
Derivatives - Other	0	0
Total Liabilities	$ 11	$ 8