FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details) - Forward Contracts - Level 3 $ in Thousands	Sep. 30, 2019 USD ($) $ / decatherm	Dec. 31, 2018 USD ($) $ / decatherm
Natural Gas | Minimum
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	1.69	1.56
Natural Gas | Minimum | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	1.78	3.13
Natural Gas | Maximum
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	8.31	9.00
Natural Gas | Maximum | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	5.78	6.00
Natural Gas | Weighted Average
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.50	3.12
Natural Gas | Weighted Average | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.66	4.53
Electricity | Minimum | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed Electric Load Profile	40.24%	0.00%
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	59.76%	100.00%
Electricity | Weighted Average | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed Electric Load Profile	54.98%	54.55%
Electricity | Weighted Average | Off-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed Electric Load Profile	45.02%	45.45%
Discounted Cash Flow | Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 23,490	$ 20,706
Liabilities | $	12,719	8,976
Discounted Cash Flow | Natural Gas | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	8,947	5,005
Liabilities | $	706	77
Discounted Cash Flow | Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	2,332	7,100
Liabilities | $	$ 1,242	$ 2,769