LONG-TERM DEBT (Details) - USD ($)	1 Months Ended				12 Months Ended
	Oct. 31, 2017	Jul. 31, 2017	May 31, 2017	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2018	Nov. 30, 2017	Aug. 31, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 1,204,173,000	$ 1,047,482,000
Less SJI Consolidated Current Maturities (all at SJG)					(63,809,000)	(231,909,000)
Total Long-Term Debt					1,140,364,000	815,573,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2018					63,809,000
2019					458,909,000
2020					67,909,000
2021					27,909,000
2022					66,084,000
Amount of facility					660,000,000
Unamortized debt issuance costs					17,400,000	7,600,000
Interest Expense					$ 54,019,000	31,449,000	$ 31,622,000
Senior notes | Series Notes at variable rates due 2019 (Q)
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate at period end					3.01%
South Jersey Gas Company
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 829,173,000	645,082,000
Less SJI Consolidated Current Maturities (all at SJG)					(63,809,000)	(215,909,000)
Total Long-Term Debt					765,364,000	429,173,000
Long-term Debt, Fiscal Year Maturity [Abstract]
2018					63,809,000
2019					218,909,000
2020					17,909,000
2021					27,909,000
2022					31,084,000
Amount of facility					210,000,000
Unamortized debt issuance costs					7,300,000	6,000,000
Interest Expense					24,705,000	17,875,000	$ 19,906,000
South Jersey Gas Company | Term facility
Long-term Debt, Fiscal Year Maturity [Abstract]
Amount of facility					10,000,000
South Jersey Gas Company | Unsecured term loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Line of credit balance					$ 200,000,000
South Jersey Gas Company | Credit Agreement | Unsecured term loan
Long-term Debt, Fiscal Year Maturity [Abstract]
Amount of facility				$ 200,000,000.0
South Jersey Gas Company | First Mortgage Bonds | Series due 2017 (C)
Debt Instrument [Line Items]
Stated interest rate					4.657%
Total Long-Term Debt Outstanding					$ 0	15,000,000
South Jersey Gas Company | First Mortgage Bonds | Series Due 2016, 4.6 Percent
Debt Instrument [Line Items]
Stated interest rate		4.657%
Long-term Debt, Fiscal Year Maturity [Abstract]
Amount of debt retired		$ 15,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2018
Debt Instrument [Line Items]
Stated interest rate					7.97%
Total Long-Term Debt Outstanding					$ 10,000,000	10,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2018
Debt Instrument [Line Items]
Stated interest rate					7.125%
Total Long-Term Debt Outstanding					$ 20,000,000	20,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2019
Debt Instrument [Line Items]
Stated interest rate					5.587%
Total Long-Term Debt Outstanding					$ 10,000,000	10,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2024 (D)
Debt Instrument [Line Items]
Stated interest rate					3.00%
Total Long-Term Debt Outstanding					$ 50,000,000	50,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Annual payment					$ 10,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2024 (E)
Debt Instrument [Line Items]
Stated interest rate					3.03%
Total Long-Term Debt Outstanding					$ 35,000,000	35,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Annual payment					$ 7,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2025 (F)
Debt Instrument [Line Items]
Stated interest rate					3.63%
Total Long-Term Debt Outstanding					$ 7,273,000	8,182,000
Less SJI Consolidated Current Maturities (all at SJG)					(900,000)
Long-term Debt, Fiscal Year Maturity [Abstract]
Payment towards principal					$ 900,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2026 (G)
Debt Instrument [Line Items]
Stated interest rate					4.84%
Total Long-Term Debt Outstanding					$ 15,000,000	15,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Annual payment					$ 2,500,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2026 (H)
Debt Instrument [Line Items]
Stated interest rate					4.93%
Total Long-Term Debt Outstanding					$ 45,000,000	45,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Annual payment					$ 7,500,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2027
Debt Instrument [Line Items]
Stated interest rate					4.03%
Total Long-Term Debt Outstanding					$ 45,000,000	45,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2030 (I)
Debt Instrument [Line Items]
Stated interest rate					4.01%
Total Long-Term Debt Outstanding					$ 50,000,000	50,000,000
South Jersey Gas Company | First Mortgage Bonds | MTN, 4.01%, Due November 2018 and November 2019
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					8,000,000
South Jersey Gas Company | First Mortgage Bonds | MTN, 4.01%, due November 2025
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					2,000,000
South Jersey Gas Company | First Mortgage Bonds | MTN, 4.01%, due November 2026
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					3,000,000
South Jersey Gas Company | First Mortgage Bonds | MTN, 4.01%, due November 2027
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					8,000,000
South Jersey Gas Company | First Mortgage Bonds | MTN, 4.01%, due November 2028, November 2029 and November 2030
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 7,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2030
Debt Instrument [Line Items]
Stated interest rate					4.23%
Total Long-Term Debt Outstanding					$ 30,000,000	30,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2032 (J)
Debt Instrument [Line Items]
Stated interest rate					3.74%
Total Long-Term Debt Outstanding					$ 35,000,000	35,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Annual payment					$ 3,175,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2033
Debt Instrument [Line Items]
Stated interest rate					5.55%
Total Long-Term Debt Outstanding					$ 32,000,000	32,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2034
Debt Instrument [Line Items]
Stated interest rate					6.213%
Total Long-Term Debt Outstanding					$ 10,000,000	10,000,000
South Jersey Gas Company | First Mortgage Bonds | Series due 2035
Debt Instrument [Line Items]
Stated interest rate					5.45%
Total Long-Term Debt Outstanding					$ 10,000,000	10,000,000
South Jersey Gas Company | First Mortgage Bonds | Series E, 2017, due January 2047
Debt Instrument [Line Items]
Stated interest rate					3.00%
Total Long-Term Debt Outstanding					$ 200,000,000	0
South Jersey Gas Company | Unsecured debt | Series A 2006 Bonds at variable rates due 2036
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 24,900,000	24,900,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate at period end					1.66%
South Jersey Gas Company | Loans payable | Term facility
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 200,000,000	200,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Repayments of multiple-draw term facility				$ 200,000,000
Amount of facility					200,000,000
South Jersey Gas Company | Medium-term notes | Series E, 2017, due January 2047
Debt Instrument [Line Items]
Stated interest rate				3.00%
Long-term Debt, Fiscal Year Maturity [Abstract]
Private placement				$ 200,000,000.0
Marina Energy LLC | Unsecured debt
Long-term Debt, Fiscal Year Maturity [Abstract]
Amount of debt retired			$ 62,300,000
Redemption amount			61,400,000
Marina Energy LLC | Unsecured debt | Series A 2001 Bonds at variable rates due 2031
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					0	20,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Redemption amount			20,000,000
Marina Energy LLC | Unsecured debt | Series B 2001 Bonds at variable rates due 2021
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					0	25,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Redemption amount			25,000,000
Marina Energy LLC | Unsecured debt | Series A 2006 Bonds at variable rates due 2036
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					0	16,400,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Redemption amount			$ 16,400,000
South Jersey Industries Inc.
Long-term Debt, Fiscal Year Maturity [Abstract]
Amount of facility					450,000,000
South Jersey Industries Inc. | Unsecured debt | South Jersey Industries Term Loan at variable rates due 2020 (R)
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 50,000,000	50,000,000
South Jersey Industries Inc. | Unsecured debt | Term loan due 2015 | Term facility
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate at period end					2.29%
South Jersey Industries Inc. | Senior notes
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding										$ 50,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Private placement										$ 100,000,000.0
South Jersey Industries Inc. | Senior notes | Series B 2012 Notes due 2017 (O)
Debt Instrument [Line Items]
Stated interest rate					2.71%						2.71%
Total Long-Term Debt Outstanding					$ 0	16,000,000
Less SJI Consolidated Current Maturities (all at SJG)											$ (16,000,000)
South Jersey Industries Inc. | Senior notes | Series due 2019
Debt Instrument [Line Items]
Stated interest rate					3.05%
Total Long-Term Debt Outstanding					$ 60,000,000	60,000,000
South Jersey Industries Inc. | Senior notes | Series due 2019
Debt Instrument [Line Items]
Stated interest rate					3.05%
Total Long-Term Debt Outstanding					$ 30,000,000	30,000,000
South Jersey Industries Inc. | Senior notes | Series due 2019
Debt Instrument [Line Items]
Stated interest rate					3.05%
Total Long-Term Debt Outstanding					$ 50,000,000	50,000,000
South Jersey Industries Inc. | Senior notes | Series C 2012 Notes due 2022
Debt Instrument [Line Items]
Stated interest rate					3.46%
Total Long-Term Debt Outstanding					$ 35,000,000	35,000,000
South Jersey Industries Inc. | Senior notes | Series 2017A-1, Due August 2024
Debt Instrument [Line Items]
Stated interest rate					3.22%
Total Long-Term Debt Outstanding					$ 25,000,000	0
South Jersey Industries Inc. | Senior notes | Series 2017B-1, Due August 2027
Debt Instrument [Line Items]
Stated interest rate					3.46%
Total Long-Term Debt Outstanding					$ 25,000,000	0
South Jersey Industries Inc. | Senior notes | Series Notes at variable rates due 2019 (Q)
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					40,000,000	40,000,000
South Jersey Industries Inc. | Senior notes | Series Notes at variable rates due 2019 (Q)
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding					$ 60,000,000	$ 60,000,000
Subsequent event | South Jersey Industries Inc. | Senior notes
Debt Instrument [Line Items]
Total Long-Term Debt Outstanding								$ 50,000,000
Elizabethtown Gas and Elkton Gas
Long-term Debt, Fiscal Year Maturity [Abstract]
Unamortized debt issuance costs	$ 12,900,000
Amortization of debt issuance costs	10,400,000
Interest Expense	$ 2,600,000
Bridge Loan | Elizabethtown Gas and Elkton Gas
Long-term Debt, Fiscal Year Maturity [Abstract]
Term of agreement	364 days
Short-term Debt	$ 2,600,000,000								$ 1,700,000,000