SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) category	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 521,945	$ 425,829
Total Operating Income	157,870	68,864
Total Depreciation and Amortization	33,898	29,521
Total Interest Charges	13,972	16,745
Total Income Taxes	36,415	21,870
Total Property Additions	55,084	70,178
Total Identifiable Assets	3,890,080		$ 3,865,086
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	$ 1,751		1,757
Energy Group and Energy Services
Segment Reporting Information [Line Items]
Number of operating categories | category	2
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 543,715	443,032
Total Interest Charges	18,617	20,077
Operating Segments | Gas Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	234,459	196,814
Total Operating Income	92,801	81,225
Total Depreciation and Amortization	20,315	17,362
Total Interest Charges	6,728	5,878
Total Income Taxes	21,836	29,911
Total Property Additions	50,237	62,280
Total Identifiable Assets	2,930,127		2,865,974
Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues	274,579	213,352
Total Operating Income	69,691	(11,987)
Total Depreciation and Amortization	98	111
Total Interest Charges	146	3,144
Total Income Taxes	17,535	(6,231)
Total Property Additions	178	298
Total Identifiable Assets	239,976		300,643
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	190,343	127,517
Total Operating Income	75,657	(11,626)
Total Depreciation and Amortization	23	28
Total Interest Charges	0	3,059
Total Income Taxes	19,127	(6,319)
Total Property Additions	5	3
Total Identifiable Assets	150,330		208,785
Operating Segments | Energy Group | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	40,201	36,878
Total Operating Income	(5,758)	(1,667)
Total Depreciation and Amortization	75	83
Total Interest Charges	146	85
Total Income Taxes	(1,534)	(447)
Total Property Additions	173	295
Total Identifiable Assets	57,497		56,935
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	44,035	48,957
Total Operating Income	(208)	1,306
Total Income Taxes	(58)	535
Total Identifiable Assets	32,149		34,923
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues	21,677	21,270
Total Operating Income	(51)	(2,041)
Total Depreciation and Amortization	10,271	11,647
Total Income Taxes	(1,026)	(3,086)
Total Property Additions	1,113	7,355
Total Identifiable Assets	584,840		583,925
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	21,157	19,612
Total Operating Income	(554)	(1,969)
Total Depreciation and Amortization	10,271	11,593
Total Interest Charges	3,847	5,814
Total Income Taxes	(1,157)	(3,069)
Total Property Additions	1,113	7,349
Total Identifiable Assets	584,349		582,587
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	520	1,658
Total Operating Income	503	(72)
Total Depreciation and Amortization	0	54
Total Income Taxes	131	(17)
Total Property Additions	0	6
Total Identifiable Assets	491		1,338
Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	13,000	11,596
Total Operating Income	(4,571)	1,667
Total Depreciation and Amortization	3,214	401
Total Interest Charges	7,470	4,731
Total Income Taxes	(1,992)	1,360
Total Property Additions	3,345	93
Total Identifiable Assets	640,996		711,038
Intersegment eliminations
Segment Reporting Information [Line Items]
Total Operating Revenues	(21,770)	(17,203)
Total Interest Charges	(4,645)	(3,332)
Total Identifiable Assets	(572,441)		(661,363)
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	64,831		$ 63,112
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	426	510
Total Income Taxes	62	(84)
Total Property Additions	$ 211	$ 152