FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details) - Forward Contracts - Discounted Cash Flow - Level 3 $ in Thousands	Mar. 31, 2021 USD ($) $ / decatherm	Dec. 31, 2020 USD ($) $ / decatherm
Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 12,367	$ 12,824
Liabilities | $	826	1,764
Natural Gas | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	3,402	3,393
Liabilities | $	$ 11	$ 8
Natural Gas | Minimum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	1.33	1.44
Natural Gas | Minimum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	1.75	2.48
Natural Gas | Maximum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	6.61	6.77
Natural Gas | Maximum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	5.03	3.63
Natural Gas | Weighted Average | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	2.61	2.67
Natural Gas | Weighted Average | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	3.12	3.16
Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 205	$ 576
Liabilities | $	$ 326	$ 630
Electricity | Minimum | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.4034	0.4034
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	0.5966
Electricity | Weighted Average | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.7051	0.6569
Electricity | Weighted Average | Fixed electric load profile (off-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.2949	0.3431