LONG-TERM DEBT (Details)		1 Months Ended
	Apr. 01, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) tranche	Apr. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Mar. 25, 2021 USD ($)	Dec. 31, 2018
Debt Instrument [Line Items]
Gross proceeds from public offering					$ 42,300,000
Net proceeds from public offering, after underwriting discounts and commissions					40,600,000
Principal amount issued			$ 250,000,000
2018 Series A Remarketable Junior Notes due 2031
Debt Instrument [Line Items]
Subordinated notes					287,500,000	$ 287,500,000
Interest rate						5.02%	3.70%
Series 2018A, Due 2021
Debt Instrument [Line Items]
Interest rate				3.43%
Repayments of senior debt				$ 90,000,000
SJG | Mortgages
Debt Instrument [Line Items]
Principal amount issued			$ 125,000,000
Number of tranches | tranche			3
SJG | Mortgages | Tranche A
Debt Instrument [Line Items]
Interest rate			2.26%
Principal amount issued			$ 50,000,000
SJG | Mortgages | Tranche B
Debt Instrument [Line Items]
Interest rate			3.08%
Principal amount issued			$ 25,000,000
SJG | Mortgages | Tranche C
Debt Instrument [Line Items]
Interest rate			3.36%
Principal amount issued			$ 50,000,000
SJG | MTNs With Annual Payment Due Beginning In 2021 | Mortgages
Debt Instrument [Line Items]
Annual payment		$ 10,000,000	$ 7,500,000		$ 2,500,000
Interest rate		3.00%	4.93%		4.84%
Corporate Units
Debt Instrument [Line Items]
Gross proceeds from public offering					$ 300,000,000
Net proceeds from public offering, after underwriting discounts and commissions					$ 291,000,000
Corporate Units | Over-Allotment Option
Debt Instrument [Line Items]
Gross proceeds from public offering	$ 35,000,000
Net proceeds from public offering, after underwriting discounts and commissions	$ 34,000,000