LINES OF CREDIT AND SHORT-TERM BORROWINGS - Schedule of Lines of Credit (Details)	Mar. 31, 2021 USD ($)
Line of Credit
Line of Credit Facility [Line Items]
Total Facility	$ 910,000,000
Usage	183,200,000
Available Liquidity	726,800,000
SJI | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	500,000,000
Usage	136,700,000
Available Liquidity	363,300,000
SJI | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	500,000,000
Usage	136,700,000
Available Liquidity	363,300,000
Letters of credit outstanding	9,500,000
SJI | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	500,000,000.0
SJG | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	210,000,000
Usage	1,400,000
Available Liquidity	208,600,000
SJG | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	1,400,000
SJG | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000.0
SJG | Line of Credit | Uncommitted Bank Line
Line of Credit Facility [Line Items]
Total Facility	10,000,000
Usage	0
Available Liquidity	10,000,000
SJG | Commercial Paper and Letters of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000
Usage	1,400,000
Available Liquidity	198,600,000
ETG/SJIU | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	1,000,000.0
ETG/SJIU | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000
Usage	45,100,000
Available Liquidity	$ 154,900,000