DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MMcfe in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) MMcfe	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments			$ (15,300,000)	$ (1,600,000)
Energy Related Derivative
Derivative [Line Items]
Unrealized gains (losses)			27,800,000		$ 2,400,000
Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments	$ (15,230,000)	$ (1,285,000)	$ (15,274,000)	$ (1,561,000)
Derivatives not designated as hedging instruments under GAAP | Expected future purchases of natural gas (in MMdts) | Purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			68,700
Derivatives not designated as hedging instruments under GAAP | Expected future purchases of natural gas (in MMdts) | Sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			105,500
Derivatives not designated as hedging instruments under GAAP | Basis and Index related net purchase contracts (in MMdts)
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			66,600
SJG | Energy Related Derivative
Derivative [Line Items]
Unrealized gains (losses)			$ 13,500,000		$ 1,100,000
SJG | Interest Rate Swap, $12,500,000 Contract 1
Derivative [Line Items]
Notional Amount	$ 12,500,000		$ 12,500,000
Fixed Interest Rate	3.53%		3.53%
SJG | Interest Rate Swap, $12,500,000 Contract 2
Derivative [Line Items]
Notional Amount	$ 12,500,000		$ 12,500,000
Fixed Interest Rate	3.43%		3.43%
SJG | Derivatives not designated as hedging instruments under GAAP | Expected future purchases of natural gas (in MMdts) | Purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			10,800
SJG | Derivatives not designated as hedging instruments under GAAP | Expected future purchases of natural gas (in MMdts) | Sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			800
SJG | Derivatives not designated as hedging instruments under GAAP | Basis and Index related net purchase contracts (in MMdts)
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			10,600
SJE and SJRG | Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments	$ (15,200,000)	$ (1,300,000)