FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details) - Forward Contracts - Fair Value, Inputs, Level 3 $ in Thousands	Mar. 31, 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	1.87	1.56
Natural Gas | Minimum | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.28	3.13
Natural Gas | Maximum
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	8.88	9
Natural Gas | Maximum | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	6.39	6
Natural Gas | Weighted Average
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.84	3.12
Natural Gas | Weighted Average | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Forward price (in dollars per dt) | $ / decatherm	2.97	4.53
Electricity | Minimum | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	37.45%	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	62.55%	100.00%
Electricity | Weighted Average | On-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	54.68%	54.55%
Electricity | Weighted Average | Off-Peak
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Fixed electric load profile	45.32%	45.45%
Discounted Cash Flow | Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 17,342	$ 20,706
Liabilities | $	12,019	8,976
Discounted Cash Flow | Natural Gas | SJG Utility Operations
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	1,709	5,005
Liabilities | $	3	77
Discounted Cash Flow | Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	4,125	7,100
Liabilities | $	$ 1,171	$ 2,769