LINES OF CREDIT - Schedule of Lines of Credit (Details) - USD ($)	Mar. 31, 2020	Mar. 26, 2020	Sep. 23, 2019
Line of Credit
Line of Credit Facility [Line Items]
Total Facility	$ 1,210,000,000
Usage	708,700,000
Available Liquidity	501,300,000
SJI | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	800,000,000
Usage	469,300,000
Available Liquidity	330,700,000
SJI | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	50,000,000
Usage	50,000,000
Available Liquidity	0
SJI | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Total Facility	500,000,000
Usage	169,300,000
Available Liquidity	330,700,000
Letters of credit outstanding	9,600,000
SJI | Unsecured Debt | Unsecured Term Loan
Line of Credit Facility [Line Items]
Total Facility	100,000,000		$ 100,000,000.0
Usage	100,000,000
Available Liquidity	0
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	143,300,000
Available Liquidity	66,700,000
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	143,300,000
Available Liquidity	56,700,000
ETG/ELK | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility	200,000,000.0
Usage	96,100,000
Available Liquidity	103,900,000
ETG/ELK | Line of Credit | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 1,000,000.0