SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) project	Mar. 31, 2019 USD ($) project	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of solar energy sites expected to be sold | project	3
Number of projects closed | project	1	4
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 534,112	$ 637,298
Total Operating Income	165,762	134,553
Total Depreciation and Amortization	35,902	31,991
Total Interest Charges	32,536	28,653
Total Income Taxes	33,370	24,949
Total Property Additions	108,394	94,594
Total Identifiable Assets	6,294,490		$ 6,365,340
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,754		1,766
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	549,825	649,676
Total Interest Charges	34,768	32,408
Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	387,969	415,746
Total Operating Income	159,242	143,124
Total Depreciation and Amortization	34,643	29,472
Total Interest Charges	14,699	14,175
Total Income Taxes	35,988	30,762
Total Property Additions	107,635	93,974
Total Identifiable Assets	5,899,921		5,829,124
Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues	140,669	212,698
Total Operating Income	6,191	(4,162)
Total Depreciation and Amortization	16	23
Total Income Taxes	1,803	(726)
Total Identifiable Assets	183,353		225,927
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	128,444	190,007
Total Operating Income	7,412	(2,506)
Total Depreciation and Amortization	16	23
Total Income Taxes	2,006	(477)
Total Identifiable Assets	157,548		195,576
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	12,225	22,691
Total Operating Income	(1,221)	(1,656)
Total Income Taxes	(203)	(249)
Total Identifiable Assets	25,805		30,351
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues	7,477	11,861
Total Operating Income	143	713
Total Depreciation and Amortization	3	1,252
Total Income Taxes	1,216	(311)
Total Property Additions	53	23
Total Identifiable Assets	47,106		154,021
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	6,988	11,330
Total Operating Income	(296)	118
Total Depreciation and Amortization	3	1,252
Total Interest Charges	1,602	2,285
Total Income Taxes	1,049	(478)
Total Property Additions	53	23
Total Identifiable Assets	46,865		154,021
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	489	531
Total Operating Income	439	595
Total Income Taxes	167	167
Total Identifiable Assets	241		0
Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	13,710	9,371
Total Operating Income	186	(5,122)
Total Depreciation and Amortization	1,240	1,244
Total Interest Charges	17,887	15,404
Total Income Taxes	(5,608)	(4,744)
Total Property Additions	661	585
Total Identifiable Assets	272,642		403,170
Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues	(15,713)	(12,378)
Total Interest Charges	(2,232)	(3,755)
Total Identifiable Assets	(196,275)		(332,185)
ETG Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	144,157	140,174
Total Operating Income	54,062	44,149
Total Depreciation and Amortization	9,451	6,658
Total Interest Charges	7,145	6,321
Total Income Taxes	10,621	6,902
Total Property Additions	49,014	38,024
Total Identifiable Assets	2,491,100		2,458,846
ELK Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	3,118	3,374
Total Operating Income	535	630
Total Depreciation and Amortization	133	112
Total Interest Charges	12	6
Total Income Taxes	136	163
Total Property Additions	651	645
Total Identifiable Assets	21,466		21,723
SJG
Segment Reporting Information [Line Items]
Total Operating Revenues		272,198
Total Operating Income	104,645	98,345
Total Income Taxes	25,231	23,697
Total Identifiable Assets	3,387,355		3,348,555
SJG | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	240,694	272,198
Total Operating Income	104,645	98,345
Total Depreciation and Amortization	25,059	22,702
Total Interest Charges	7,542	7,848
Total Income Taxes	25,231	23,697
Total Property Additions	57,970	55,305
Total Identifiable Assets	3,387,355		3,348,555
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	85,989		$ 83,517
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	580	544
Total Income Taxes	(29)	(32)
Total Property Additions	$ 45	$ 12