DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MWh in Millions, MMcfe in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($) MWh	Mar. 31, 2019 USD ($) MMcfe	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Unrealized gains (losses)	$ 2,200,000				$ 4,100,000
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			15.5
Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments	$ (12,060,000)			$ 23,353,000
Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		1.4	81.0
Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		1.1	76.8
SJG
Derivative [Line Items]
Unrealized gains (losses)	1,300,000			$ 3,300,000
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 | Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		0.0	13.5
SJG | Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)		0.0	0.3
SJG | Basis and Index related net purchase (sales) contracts
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe			2.5