LINES OF CREDIT AND SHORT-TERM BORROWINGS - Schedule of Lines of Credit (Details) - USD ($)	Sep. 30, 2020	Mar. 26, 2020
Line of Credit
Line of Credit Facility [Line Items]
Total Facility	$ 1,060,000,000
Usage	608,400,000
Available Liquidity	451,600,000
SJI | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	650,000,000
Usage	349,300,000
Available Liquidity	300,700,000
SJI | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	500,000,000.0
Usage	199,300,000
Available Liquidity	300,700,000
Letters of credit outstanding	9,600,000
SJI | Unsecured Debt | Unsecured Term Loan
Line of Credit Facility [Line Items]
Total Facility	150,000,000	$ 150,000,000.0
Usage	150,000,000
Available Liquidity	0
SJG | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	210,000,000
Usage	108,700,000
Available Liquidity	101,300,000
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 | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	800,000
SJG | Commercial Paper and Letters of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000
Usage	108,700,000
Available Liquidity	91,300,000
ETG/SJIU | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000
Usage	150,400,000
Available Liquidity	49,600,000
ETG/SJIU | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 1,000,000.0