LINES OF CREDIT (Details)			9 Months Ended
	Jul. 01, 2018 USD ($)	Jun. 26, 2018 USD ($) Covenant	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Payment to acquire assets			$ 200,770,000	$ 205,604,000
SJG Utility Operations
Line of Credit Facility [Line Items]
Payment to acquire assets			168,654,000	$ 183,875,000
Commercial Paper and Letters of Credit | SJG Utility Operations | Revolving Credit Facility
Line of Credit Facility [Line Items]
Usage			141,000,000
Line of Credit
Line of Credit Facility [Line Items]
Total Facility			860,000,000
Usage			428,300,000
Available Liquidity			$ 431,700,000
Weighted average interest rate on borrowings			2.68%	2.26%
Average borrowings outstanding			$ 258,800,000	$ 260,900,000
Debt covenant, ratio of indebtedness to total capitalization, syndicate			0.70
Proceeds from issuance of equity			$ 500,000,000
Debt covenant, ratio of indebtedness to total capitalization			0.75
Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Letters of Credit, Maximum Aggregate Amount		$ 50,000,000
Line of Credit | SJI
Line of Credit Facility [Line Items]
Total Facility			$ 450,000,000
Usage			259,500,000
Available Liquidity			190,500,000
Line of Credit | SJI | Line of Credit
Line of Credit Facility [Line Items]
Total Facility			400,000,000
Usage			209,500,000
Available Liquidity			190,500,000
Letters of credit outstanding			6,100,000
Line of Credit | SJI | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility			50,000,000
Usage			50,000,000
Available Liquidity			0
Line of Credit | Elizabethtown Gas | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility		250,000,000
Additional aggregate borrowing capacity		175,000,000
Letters of Credit, Maximum Aggregate Amount		50,000,000
Line of Credit | Elkton Gas | Revolving Credit Facility
Line of Credit Facility [Line Items]
Additional aggregate borrowing capacity		25,000,000
Line of Credit | ETG Utility Operations | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility		$ 200,000,000.0	200,000,000
Term (in years)		2 years
Additional aggregate borrowing capacity		$ 200,000,000
Number of Financial Covenants | Covenant		1
Usage			27,800,000
Available Liquidity			172,200,000
Line of Credit | ETG Utility Operations | Swingline Loan [Member]
Line of Credit Facility [Line Items]
Additional aggregate borrowing capacity		$ 20,000,000
Line of Credit | SJG Utility Operations
Line of Credit Facility [Line Items]
Total Facility			210,000,000
Usage			141,000,000
Available Liquidity			$ 69,000,000
Weighted average interest rate on borrowings			2.42%
Average borrowings outstanding			$ 67,900,000	$ 17,000,000
Debt covenant, ratio of indebtedness to total capitalization			0.65
Line of Credit | SJG Utility Operations | Line of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding			$ 800,000
Line of Credit | SJG Utility Operations | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total Facility			200,000,000
Available Liquidity			59,000,000
Line of Credit | SJG Utility Operations | Uncommitted Bank Line
Line of Credit Facility [Line Items]
Total Facility			10,000,000
Usage			0
Available Liquidity			10,000,000
Unsecured promissory notes
Line of Credit Facility [Line Items]
Principal amount issued					$ 250,000,000.0
Unsecured promissory notes | SJG Commercial Paper Program
Line of Credit Facility [Line Items]
Total Facility			$ 200,000,000
Fixed maturities of notes, at maximum number of days			270 days
Commercial Paper | SJG Utility Operations | Revolving Credit Facility
Line of Credit Facility [Line Items]
Usage			$ 140,200,000
ETG Utility Operations
Line of Credit Facility [Line Items]
Payment to acquire assets	$ 1,700,000,000