LINES OF CREDIT (Details)	1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Credit Facilities and Available Liquidity [Abstract]
Maximum amount outstanding on lines of credit during the period				$ 404,800,000	$ 431,000,000
SJG Utility Operations
Credit Facilities and Available Liquidity [Abstract]
Maximum amount outstanding on lines of credit during the period		$ 108,000,000	$ 85,000,000
Commercial Paper and Letters of Credit | SJG Utility Operations | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Usage		69,300,000
Line of Credit
Credit Facilities and Available Liquidity [Abstract]
Total Facility		860,000,000
Usage		366,500,000
Available Liquidity		$ 493,500,000
Weighted average interest rate on borrowings		3.46%	2.67%
Average borrowings outstanding		$ 241,900,000	$ 238,000,000
Debt covenant, ratio of indebtedness to total capitalization, syndicate	0.70	0.70
Line of Credit | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Maximum aggregate letters of credit allowed	$ 50,000,000
Line of Credit | SJI
Credit Facilities and Available Liquidity [Abstract]
Total Facility		$ 450,000,000
Usage		202,200,000
Available Liquidity		247,800,000
Line of Credit | SJI | Line of Credit
Credit Facilities and Available Liquidity [Abstract]
Total Facility		400,000,000
Usage		152,200,000
Available Liquidity		247,800,000
Letters of credit outstanding		10,600,000
Line of Credit | SJI | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Total Facility		50,000,000
Usage		50,000,000
Available Liquidity		0
Line of Credit | Elizabethtown Gas | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Total Facility	250,000,000.0
Additional aggregate borrowing capacity	175,000,000
Maximum aggregate letters of credit allowed	50,000,000
Line of Credit | Elkton Gas | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Additional aggregate borrowing capacity	25,000,000
Line of Credit | ETG Utility Operations | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Total Facility	$ 200,000,000.0	200,000,000
Usage		95,000,000
Available Liquidity		105,000,000
Term (in years)	2 years
Additional aggregate borrowing capacity	$ 200,000,000
Line of Credit | ETG Utility Operations | Swingline Loan
Credit Facilities and Available Liquidity [Abstract]
Additional aggregate borrowing capacity	$ 20,000,000
Line of Credit | SJG Utility Operations
Credit Facilities and Available Liquidity [Abstract]
Total Facility		210,000,000
Usage		69,300,000
Available Liquidity		$ 140,700,000
Weighted average interest rate on borrowings		2.74%	2.32%
Average borrowings outstanding		$ 84,600,000	$ 47,700,000
Debt covenant, ratio of indebtedness to total capitalization		0.65
Line of Credit | SJG Utility Operations | Line of Credit
Credit Facilities and Available Liquidity [Abstract]
Letters of credit outstanding		$ 800,000
Line of Credit | SJG Utility Operations | Revolving Credit Facility
Credit Facilities and Available Liquidity [Abstract]
Total Facility		200,000,000
Available Liquidity		130,700,000
Line of Credit | SJG Utility Operations | Uncommitted Bank Line
Credit Facilities and Available Liquidity [Abstract]
Total Facility		10,000,000
Usage		0
Available Liquidity		10,000,000
Unsecured promissory notes | SJG Commercial Paper Program
Credit Facilities and Available Liquidity [Abstract]
Total Facility		$ 200,000,000
Fixed maturities of notes, at maximum number of days		270 days