DERIVATIVE INSTRUMENTS - Fair Value of all Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Energy-related commodity contracts
Derivatives, Fair Value [Line Items]
Assets	$ 54,625	$ 48,374
Liabilities	0	0
Energy-related commodity contracts | SJG
Derivatives, Fair Value [Line Items]
Assets	4,382	4,140
Liabilities	0	0
Derivatives not designated as hedging instruments under GAAP
Derivatives, Fair Value [Line Items]
Assets	54,625	48,374
Liabilities	41,096	41,891
Derivatives not designated as hedging instruments under GAAP | SJG
Derivatives, Fair Value [Line Items]
Assets	4,382	4,140
Liabilities	8,068	12,996
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Current
Derivatives, Fair Value [Line Items]
Assets	38,541	41,439
Liabilities	21,837	27,006
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Current | SJG
Derivatives, Fair Value [Line Items]
Assets	4,382	4,053
Liabilities	352	2,868
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Non-Current
Derivatives, Fair Value [Line Items]
Assets	16,084	6,935
Liabilities	11,818	4,947
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Non-Current | SJG
Derivatives, Fair Value [Line Items]
Assets	0	87
Liabilities	275	190
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Current
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	502	659
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Current | SJG
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	502	659
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Noncurrent
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	6,939	9,279
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Noncurrent | SJG
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	$ 6,939	$ 9,279