DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MMcfe in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MMcfe	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments			$ (35,100,000)	$ (7,800,000)
Energy Related Derivative
Derivative [Line Items]
Unrealized gains (losses)			68,200,000		$ 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	$ (19,815,000)	$ (6,246,000)	$ (35,089,000)	$ (7,807,000)
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			72,800
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			116,800
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			82,600
SJG | Energy Related Derivative
Derivative [Line Items]
Unrealized gains (losses)			$ 29,400,000		$ 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			12,500
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			400
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			1,300
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	$ (19,800,000)	$ (6,200,000)