DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details) MWh in Millions	1 Months Ended	3 Months Ended		6 Months Ended				12 Months Ended
	Sep. 30, 2005 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) MWh	Jun. 30, 2017 USD ($) MMcfe	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Net realized gains (losses), derivative instruments, energy-related contracts				$ 1,300,000				$ (2,800,000)
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						112,900,000
Derivatives not designated as hedging instruments under GAAP
Derivative [Line Items]
Gain (loss) on energy related derivative instruments not designated as hedging instruments		$ (7,855,000)	$ (21,371,000)	$ 6,832,000			$ (2,716,000)
Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					2.7	50,900,000
Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					2.3	46,700,000
SJG
Derivative [Line Items]
Unrealized gains (losses)				5,800,000				(4,400,000)
Cost of Treasury Locks	$ 1,400,000
Amortized life of the associated debt issue	30 years
Unamortized balance		800,000		800,000	$ 800,000	$ 800,000		$ 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	$ 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 | Basis and Index related net purchase (sales) contracts
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh) | MMcfe						21,800,000
SJG | Expected future purchases
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					0.0	10,800,000
SJG | Expected future sales
Derivative [Line Items]
Notional amount (natural gas in mmcfe and electricity in mwh)					0.0	100,000