DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MMcfe in Thousands, MWh in Millions	1 Months Ended	3 Months Ended		9 Months Ended				12 Months Ended
	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) MWh	Sep. 30, 2018 USD ($) MMcfe	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Unrealized gains (losses)				$ 4,700,000				$ (2,100,000)
Reclassified unrealized gain (loss) to interest income (expense)	$ (2,400,000)
Interest Rate Swap, $20,000,000 Contract 1
Derivative [Line Items]
Notional Amount		$ 20,000,000		$ 20,000,000	$ 20,000,000	$ 20,000,000
Fixed Interest Rate		3.049%		3.049%	3.049%	3.049%
Interest Rate Swap, $20,000,000 Contract 2
Derivative [Line Items]
Notional Amount		$ 20,000,000		$ 20,000,000	$ 20,000,000	$ 20,000,000
Fixed Interest Rate		3.049%		3.049%	3.049%	3.049%
Interest Rate Swap, $10,000,000
Derivative [Line Items]
Notional Amount		$ 10,000,000		$ 10,000,000	$ 10,000,000	$ 10,000,000
Fixed Interest Rate		3.049%		3.049%	3.049%	3.049%
Basis and Index related net purchase (sales) contracts
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe						87,600
Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments		$ (11,225,000)	$ (4,632,000)	$ 5,950,000			$ 2,200,000
Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					1.9	74,500
Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					1.5	53,800
SJG
Derivative [Line Items]
Unrealized gains (losses)				5,100,000				$ (2,100,000)
SJG | Interest Rate Swap, $12,500,000 Contract 1
Derivative [Line Items]
Notional Amount		$ 12,500,000		$ 12,500,000	$ 12,500,000	$ 12,500,000
Fixed Interest Rate		3.53%		3.53%	3.53%	3.53%
SJG | Interest Rate Swap, $12,500,000 Contract 2
Derivative [Line Items]
Notional Amount		$ 12,500,000		$ 12,500,000	$ 12,500,000	$ 12,500,000
Fixed Interest Rate		3.43%		3.43%	3.43%	3.43%
SJG | Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					0.0	8,900
SJG | Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					0.0	300
SJG | Basis and Index related net purchase (sales) contracts
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe						1,100