FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Available-for-Sale Securities	$ 32	$ 32
Fair Value, Measurements, Recurring
Assets
Available-for-Sale Securities	32	32
Derivatives - energy related assets	178,997	48,374
Total assets	179,029	48,406
Liabilities
Derivatives - energy related liabilities	131,368	31,953
Derivatives - other	8,027	9,938
Total liabilities	139,395	41,891
Fair Value, Measurements, Recurring | SJG
Assets
Derivatives - energy related assets	32,216	4,140
Total assets	32,216	4,140
Liabilities
Derivatives - energy related liabilities	2,834	3,058
Derivatives - other	8,027	9,938
Total liabilities	10,861	12,996
Fair Value, Measurements, Recurring | Level 1
Assets
Available-for-Sale Securities	32	32
Derivatives - energy related assets	112,683	11,447
Total assets	112,715	11,479
Liabilities
Derivatives - energy related liabilities	39,306	8,605
Derivatives - other	0	0
Total liabilities	39,306	8,605
Fair Value, Measurements, Recurring | Level 1 | SJG
Assets
Derivatives - energy related assets	21,020	715
Total assets	21,020	715
Liabilities
Derivatives - energy related liabilities	1,025	2,891
Derivatives - other	0	0
Total liabilities	1,025	2,891
Fair Value, Measurements, Recurring | Level 2
Assets
Available-for-Sale Securities	0	0
Derivatives - energy related assets	55,409	23,527
Total assets	55,409	23,527
Liabilities
Derivatives - energy related liabilities	71,031	20,954
Derivatives - other	8,027	9,938
Total liabilities	79,058	30,892
Fair Value, Measurements, Recurring | Level 2 | SJG
Assets
Derivatives - energy related assets	3,938	32
Total assets	3,938	32
Liabilities
Derivatives - energy related liabilities	1,668	159
Derivatives - other	8,027	9,938
Total liabilities	9,695	10,097
Fair Value, Measurements, Recurring | Level 3
Assets
Available-for-Sale Securities	0	0
Derivatives - energy related assets	10,905	13,400
Total assets	10,905	13,400
Liabilities
Derivatives - energy related liabilities	21,031	2,394
Derivatives - other	0	0
Total liabilities	21,031	2,394
Fair Value, Measurements, Recurring | Level 3 | SJG
Assets
Derivatives - energy related assets	7,258	3,393
Total assets	7,258	3,393
Liabilities
Derivatives - energy related liabilities	141	8
Derivatives - other	0	0
Total liabilities	$ 141	$ 8