FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details) - Forward Contracts - Discounted Cash Flow - Level 3 $ in Thousands	Sep. 30, 2020 USD ($) $ / decatherm	Dec. 31, 2019 USD ($) $ / decatherm
Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 10,403	$ 21,645
Liabilities | $	4,050	4,333
Natural Gas | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	4,304	5,049
Liabilities | $	$ 0	$ 14
Natural Gas | Minimum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	0.72	1.57
Natural Gas | Minimum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	1.05	1.85
Natural Gas | Maximum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	7.09	7.28
Natural Gas | Maximum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	5.47	3.61
Natural Gas | Weighted Average | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	3.18	2.38
Natural Gas | Weighted Average | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	4.03	3.02
Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 716	$ 3,718
Liabilities | $	$ 660	$ 3,456
Electricity | Minimum | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.4034	0.0000
Electricity | Minimum | Fixed electric load profile (off-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.0000	0.0000
Electricity | Maximum | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	1.0000	1.0000
Electricity | Maximum | Fixed electric load profile (off-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.5966	1.0000
Electricity | Weighted Average | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.6014	0.5546
Electricity | Weighted Average | Fixed electric load profile (off-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.3986	0.4454