DERIVATIVE INSTRUMENTS - Fair Value of all Derivative Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Assets	$ 56,314	$ 61,190
Liabilities	63,438	39,263
SJG
Derivatives, Fair Value [Line Items]
Assets	13,893	5,479
Liabilities	16,982	8,056
Energy-related commodity contracts
Derivatives, Fair Value [Line Items]
Assets	56,314	61,190
Liabilities	0	0
Energy-related commodity contracts | SJG
Derivatives, Fair Value [Line Items]
Assets	13,893	5,479
Liabilities	0	0
Derivatives not designated as hedging instruments under GAAP
Derivatives, Fair Value [Line Items]
Assets	56,314	61,190
Liabilities	63,438	39,263
Derivatives not designated as hedging instruments under GAAP | SJG
Derivatives, Fair Value [Line Items]
Assets	13,893	5,479
Liabilities	16,982	8,056
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Current
Derivatives, Fair Value [Line Items]
Assets	46,708	54,021
Liabilities	40,653	24,134
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Current | SJG
Derivatives, Fair Value [Line Items]
Assets	13,878	5,464
Liabilities	7,742	2,146
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Non-Current
Derivatives, Fair Value [Line Items]
Assets	9,606	7,169
Liabilities	7,812	7,256
Derivatives not designated as hedging instruments under GAAP | Energy-related commodity contracts | Derivatives - Energy Related - Non-Current | SJG
Derivatives, Fair Value [Line Items]
Assets	15	15
Liabilities	244	43
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Current
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	1,352	588
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Current | SJG
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	551	343
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Noncurrent
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	13,621	7,285
Derivatives not designated as hedging instruments under GAAP | Interest rate contracts | Derivatives - Other - Noncurrent | SJG
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	$ 8,445	$ 5,524