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, 2021 USD ($) $ / decatherm	Dec. 31, 2020 USD ($) $ / decatherm
Natural Gas
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 10,729	$ 12,824
Liabilities | $	20,745	1,764
Natural Gas | SJG
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	7,258	3,393
Liabilities | $	$ 141	$ 8
Natural Gas | Minimum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	1.85	1.44
Natural Gas | Minimum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	4.51	2.48
Natural Gas | Maximum | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	12.31	6.77
Natural Gas | Maximum | SJG | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	9.97	3.63
Natural Gas | Weighted Average | Forward price (per dt)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input | $ / decatherm	5.12	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	7.49	3.16
Electricity
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Assets | $	$ 176	$ 576
Liabilities | $	$ 286	$ 630
Electricity | Minimum | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.4516	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	0.5484	0.5966
Electricity | Weighted Average | Fixed electric load profile (on-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.9320	0.6569
Electricity | Weighted Average | Fixed electric load profile (off-peak)
Fair Value Inputs, Assets And Liabilities, Quantitative Information [Line Items]
Derivative, measurement input	0.0680	0.3431