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DERIVATIVE INSTRUMENTS - Effect of Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Derivative Instruments Not Designated as Hedging Instruments [Abstract]    
Total $ (4,322) $ (13,150)
Fair value of derivative instruments with credit-risk-related features 500  
Additional collateral, aggregate fair value 400  
Interest rate contracts | Non-Utility Revenue    
Derivatives, Fair Value [Line Items]    
Losses reclassified from AOCL into income (12)  
SJG | Interest rate contracts | Non-Utility Revenue    
Derivatives, Fair Value [Line Items]    
Losses reclassified from AOCL into income (12)  
Derivatives in Cash Flow Hedging Relationships under GAAP    
Derivatives, Fair Value [Line Items]    
Losses reclassified from AOCL into income   (12)
Derivatives in Cash Flow Hedging Relationships under GAAP | SJG    
Derivatives, Fair Value [Line Items]    
Losses reclassified from AOCL into income   (12)
Derivatives not designated as hedging instruments under GAAP    
Derivative Instruments Not Designated as Hedging Instruments [Abstract]    
(Losses) gains on energy-related commodity contracts (276) (12,060)
(Losses) gains on interest rate contracts $ (4,046) $ (1,090)