SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)						1 Months Ended		2 Months Ended	3 Months Ended			9 Months Ended
	Aug. 07, 2020 USD ($)	Jul. 31, 2020 USD ($)	Feb. 18, 2020 USD ($)	Aug. 31, 2019 USD ($)	Jul. 01, 2018 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) project shares	Aug. 21, 2020 USD ($)	Sep. 30, 2020 USD ($) project county shares	Mar. 31, 2020 project	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) county project acquisition shares	Sep. 30, 2019 USD ($) project	Dec. 31, 2020 project	Aug. 12, 2020	Aug. 06, 2020
Public Utilities, General Disclosures [Line Items]
Number of businesses acquired | acquisition												4
Income tax credits									$ 12,000,000.0		$ 0	$ 12,000,000.0	$ 0
Number of projects sold | project										1		1	7
Number of solar energy projects expected to be sold | project							3		2			2
Impairment charge									$ 0		1,300,000	$ 0	$ 1,300,000
Shares of treasury stock held (in shares) | shares							231,514		251,524			251,524
Payments to purchase asset management agreement					$ 11,300,000
Minimum annual fee for long-term asset management agreement					4,250,000
Intangible asset					19,200,000		$ 11,500,000		$ 7,700,000			$ 7,700,000
Profit sharing liabilities					$ 17,546,000		10,600,000		7,500,000			7,500,000
Current portion of long-term debt							467,909,000		142,809,000			142,809,000
Notes payable							848,700,000		597,000,000			597,000,000
Debt refinanced									600,000,000.0			600,000,000.0
EnerConnex, LLC
Public Utilities, General Disclosures [Line Items]
Remaining equity interest acquired (as a percent)	75.00%
Equity interest in acquiree previously held (as a percent)																25.00%
Consideration paid	$ 7,500,000
Solar Energy Projects Acquisition
Public Utilities, General Disclosures [Line Items]
Consideration paid								$ 3,800,000
AEP
Public Utilities, General Disclosures [Line Items]
Consideration paid				$ 4,000,000.0
Assets held for sale
Public Utilities, General Disclosures [Line Items]
Consideration received on sales or projects												7,200,000	24,300,000
Assets held for sale | MTF and ACB
Public Utilities, General Disclosures [Line Items]
Consideration received on sales or projects							100,000,000.0
Assets held for sale | Solar Assets
Public Utilities, General Disclosures [Line Items]
Nonutility property and equipment held for sale									$ 20,000,000.0			$ 20,000,000.0
Disposed of by sale | MTF and ACB
Public Utilities, General Disclosures [Line Items]
Consideration received on sales or projects			$ 97,000,000.0
Disposed of by sale | ELK Utility Operations
Public Utilities, General Disclosures [Line Items]
Consideration received on sales or projects		$ 15,600,000
SJG
Public Utilities, General Disclosures [Line Items]
Number of counties in which entity operates | county									7			7
Impairment charge									$ 0		$ 0	$ 0	$ 0
Refunds received from third party supplier						$ 10,000,000.0
Current portion of long-term debt							417,909,000		52,809,000			52,809,000
Notes payable							$ 171,300,000		107,900,000			107,900,000
Debt refinanced									400,000,000.0			400,000,000.0
SJG | VSIP
Public Utilities, General Disclosures [Line Items]
VSIP total costs												600,000
Catamaran | Annadale
Public Utilities, General Disclosures [Line Items]
Ownership interest (as a percent)															100.00%
Marina | Annadale | Annadale
Public Utilities, General Disclosures [Line Items]
Ownership by parent (as a percent)															93.00%
Marina | Annadale
Public Utilities, General Disclosures [Line Items]
Investment in joint venture									$ 4,200,000			4,200,000
South Jersey Resources Group, LLC
Public Utilities, General Disclosures [Line Items]
Refunds received from third party supplier						11,200,000
Refunds received from third party supplier, amount remaining after remittance												3,900,000
South Jersey Resources Group, LLC | Operating Revenues
Public Utilities, General Disclosures [Line Items]
Refunds received from third party supplier, amount remaining after remittance												3,800,000
South Jersey Resources Group, LLC | Other Income
Public Utilities, General Disclosures [Line Items]
Refunds received from third party supplier, amount remaining after remittance												$ 100,000
ETG Utility Operations
Public Utilities, General Disclosures [Line Items]
Number of counties in which entity operates | county									7			7
Minimum annual fee for long-term asset management agreement												$ 4,250,000
ETG Utility Operations | South Jersey Resources Group, LLC
Public Utilities, General Disclosures [Line Items]
Refunds received from third party supplier, amount remitted						$ 7,100,000
Forecast
Public Utilities, General Disclosures [Line Items]
Number of solar energy projects expected to be sold | project														2