DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MMcfe in Thousands, MWh in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) MWh	Mar. 31, 2018 USD ($) MMcfe	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
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
Fixed Interest Rate	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
Fixed Interest Rate	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
Fixed Interest Rate	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			29,000
Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments	$ 23,353,000			$ 14,688,000
Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		2.4	45,400
Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		2.0	57,000
SJG
Derivative [Line Items]
Unrealized gains (losses)	700,000				$ 2,100,000
Unamortized balance					$ 900,000
SJG | Interest Rate Swap, $12,500,000 Contract 1
Derivative [Line Items]
Notional Amount	$ 12,500,000	$ 12,500,000	$ 12,500,000
Fixed Interest Rate	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
Fixed Interest Rate	3.43%	3.43%	3.43%
SJG | Basis and Index related net purchase (sales) contracts
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			(3,100)
SJG | Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			13,300
SJG | Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			300