SEGMENTS OF BUSINESS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 637,298	$ 521,945
Total Operating Income	134,553	157,870
Total Depreciation and Amortization	31,991	33,898
Total Interest Charges	28,653	13,972
Total Income Taxes	24,949	36,415
Total Property Additions	94,594	55,084
Total Identifiable Assets	5,863,362		$ 5,956,577
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,758		1,777
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	649,676	543,715
Total Interest Charges	32,408	18,617
Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	415,746	234,459
Total Operating Income	143,124	92,801
Total Depreciation and Amortization	29,472	20,315
Total Interest Charges	14,175	6,728
Total Income Taxes	30,762	21,836
Total Property Additions	93,974	50,237
Total Identifiable Assets	5,344,988		5,282,893
Operating Segments | Energy Group:
Segment Reporting Information [Line Items]
Total Operating Revenues	212,698	274,579
Total Operating Income	(4,162)	69,691
Total Depreciation and Amortization	23	98
Total Interest Charges	0	146
Total Income Taxes	(726)	17,535
Total Property Additions	0	178
Total Identifiable Assets	215,744		318,498
Operating Segments | Energy Group: | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	190,007	190,343
Total Operating Income	(2,506)	75,657
Total Depreciation and Amortization	23	23
Total Interest Charges	0	0
Total Income Taxes	(477)	19,127
Total Property Additions	0	5
Total Identifiable Assets	183,303		266,417
Operating Segments | Energy Group: | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	0	40,201
Total Operating Income	0	(5,758)
Total Depreciation and Amortization	0	75
Total Interest Charges	0	146
Total Income Taxes	0	(1,534)
Total Property Additions	0	173
Total Identifiable Assets	337		12,736
Operating Segments | Energy Group: | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	22,691	44,035
Total Operating Income	(1,656)	(208)
Total Income Taxes	(249)	(58)
Total Identifiable Assets	32,104		39,345
Operating Segments | Energy Services:
Segment Reporting Information [Line Items]
Total Operating Revenues	11,861	21,677
Total Operating Income	713	(51)
Total Depreciation and Amortization	1,252	10,271
Total Income Taxes	(311)	(1,026)
Total Property Additions	23	1,113
Total Identifiable Assets	175,751		195,329
Operating Segments | Energy Services: | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	11,330	21,157
Total Operating Income	118	(554)
Total Depreciation and Amortization	1,252	10,271
Total Interest Charges	2,285	3,847
Total Income Taxes	(478)	(1,157)
Total Property Additions	23	1,113
Total Identifiable Assets	175,508		195,329
Operating Segments | Energy Services: | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	531	520
Total Operating Income	595	503
Total Depreciation and Amortization	0	0
Total Income Taxes	167	131
Total Property Additions	0	0
Total Identifiable Assets	243		0
Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	9,371	13,000
Total Operating Income	(5,122)	(4,571)
Total Depreciation and Amortization	1,244	3,214
Total Interest Charges	15,404	7,470
Total Income Taxes	(4,744)	(1,992)
Total Property Additions	585	3,345
Total Identifiable Assets	270,381		387,482
Intersegment Sales
Segment Reporting Information [Line Items]
Total Operating Revenues	(12,378)	(21,770)
Total Interest Charges	(3,755)	(4,645)
Total Identifiable Assets	(220,558)		(301,735)
Elizabethtown Gas | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	140,174	0
Total Operating Income	44,149	0
Total Depreciation and Amortization	6,658	0
Total Interest Charges	6,321	0
Total Income Taxes	6,902	0
Total Property Additions	38,024	0
Total Identifiable Assets	2,177,507		2,148,175
Elkton Gas | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	3,374	0
Total Operating Income	630	0
Total Depreciation and Amortization	112	0
Total Interest Charges	6	0
Total Income Taxes	163	0
Total Property Additions	645	0
Total Identifiable Assets	17,088		16,482
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	75,298		72,333
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	544	426
Total Income Taxes	(32)	62
Total Property Additions	12	211
SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	272,198	234,459
Total Operating Income	98,345	92,801
Total Income Taxes	23,697	21,836
Total Identifiable Assets	3,150,393		3,118,236
SJG Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	272,198	234,459
Total Operating Income	98,345	92,801
Total Depreciation and Amortization	22,702	20,315
Total Interest Charges	7,848	6,728
Total Income Taxes	23,697	21,836
Total Property Additions	55,305	$ 50,237
Total Identifiable Assets	$ 3,150,393		$ 3,118,236