FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details) - Forward Contracts - Fair Value, Inputs, Level 3 $ in Thousands	Sep. 30, 2018 USD ($) $ / decatherm	Dec. 31, 2017 USD ($) $ / decatherm
Natural Gas | Minimum
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	1.64	1.79
Natural Gas | Minimum | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.67	2.42
Natural Gas | Maximum
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	9.82	12.09
Natural Gas | Maximum | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	7.07	6.67
Natural Gas | Weighted Average
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	3.23	3.01
Natural Gas | Weighted Average | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	4.80	5.25
Electricity | Minimum | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	0.00%	36.36%
Electricity | Minimum | Off-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	0.00%	0.00%
Electricity | Maximum | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	100.00%	100.00%
Electricity | Maximum | Off-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	0.00%	63.64%
Electricity | Weighted Average | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	53.88%	53.39%
Electricity | Weighted Average | Off-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	46.12%	46.61%
Discounted Cash Flow | Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 13,821	$ 13,519
Liabilities | $	9,497	15,686
Discounted Cash Flow | Natural Gas | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	7,548	6,894
Liabilities | $	932	4,842
Discounted Cash Flow | Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	8,838	7,584
Liabilities | $	$ 2,858	$ 2,307