FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	$ 40	$ 40
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	40	40
Derivatives - Energy Related Assets	43,411	60,135
Total Assets	43,451	60,175
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	31,268	50,171
Derivatives - Other	20,592	12,660
Total Liabilities	51,860	62,831
Fair Value, Measurements, Recurring | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	4,842	16,909
Total Assets	4,842	16,909
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	2,651	14,766
Derivatives - Other	11,407	7,856
Total Liabilities	14,058	22,622
Fair Value, Measurements, Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	40	40
Derivatives - Energy Related Assets	10,868	16,931
Total Assets	10,908	16,971
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	15,004	34,446
Derivatives - Other	0	0
Total Liabilities	15,004	34,446
Fair Value, Measurements, Recurring | Level 1 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	369	11,860
Total Assets	369	11,860
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	2,454	14,565
Derivatives - Other	0	0
Total Liabilities	2,454	14,565
Fair Value, Measurements, Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	15,591	17,841
Total Assets	15,591	17,841
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	11,563	7,936
Derivatives - Other	20,592	12,660
Total Liabilities	32,155	20,596
Fair Value, Measurements, Recurring | Level 2 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	107	0
Total Assets	107	0
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	196	187
Derivatives - Other	11,407	7,856
Total Liabilities	11,603	8,043
Fair Value, Measurements, Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Available-for-Sale Securities	0	0
Derivatives - Energy Related Assets	16,952	25,363
Total Assets	16,952	25,363
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	4,701	7,789
Derivatives - Other	0	0
Total Liabilities	4,701	7,789
Fair Value, Measurements, Recurring | Level 3 | SJG
Assets, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Assets	4,366	5,049
Total Assets	4,366	5,049
Liabilities, Fair Value Disclosure [Abstract]
Derivatives - Energy Related Liabilities	1	14
Derivatives - Other	0	0
Total Liabilities	$ 1	$ 14