SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) project	Dec. 31, 2019 USD ($) project	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) project	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of solar energy projects expected to be sold | project					3					3
Number of projects sold | project				1
Total Operating Revenues	$ 485,758	$ 261,549	$ 259,964	$ 534,112	$ 463,191	$ 261,203	$ 266,934	$ 637,298	$ 1,541,383	$ 1,628,626	$ 1,641,338
Total Operating Income									282,222	201,205	100,745
Total Depreciation and Amortization									170,647	133,385	132,914
Total Interest Charges									118,534	114,477	90,296
Total Income Taxes	8,939	$ (19,467)	$ (178)	$ 33,370	11,683	$ (10,925)	$ (4,646)	$ 24,949	22,664	21,061	561
Total Property Additions									552,925	466,294	350,856
Total Identifiable Assets	6,689,148				6,365,340				6,689,148	6,365,340
ETG Utility Operations
Segment Reporting Information [Line Items]
Property, Plant and Equipment											1,077,000
ELK Utility Operations
Segment Reporting Information [Line Items]
Property, Plant and Equipment											12,300
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,775				1,766				1,775	1,766
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues									1,605,852	1,685,895	1,716,730
Total Interest Charges									125,836	127,629	110,417
Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues									(64,469)	(57,269)	(75,392)
Total Interest Charges									(7,302)	(13,152)	(20,121)
Total Identifiable Assets	(225,331)				(332,185)				(225,331)	(332,185)
SJG Utility Operations | Operating Segments | SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues									571,787	569,226	548,000
Total Operating Income									171,235	147,494	132,688
Total Depreciation and Amortization									101,711	93,910	82,622
Total Interest Charges									33,388	31,654	28,011
Total Income Taxes									35,324	32,822	26,413
Total Property Additions									266,009	264,235	253,617
Total Identifiable Assets	3,522,265				3,348,555				3,522,265	3,348,555
ETG Utility Operations | Operating Segments | SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues									349,392	325,133	125,604
Total Operating Income									89,638	69,315	2,164
Total Depreciation and Amortization									57,967	29,051	13,580
Total Interest Charges									29,997	27,352	10,478
Total Income Taxes									12,465	7,761	(3,086)
Total Property Additions									197,730	197,457	90,259
Total Identifiable Assets	2,561,067				2,458,846				2,561,067	2,458,846
ELK Utility Operations | Operating Segments | SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues									4,793	7,949	3,302
Total Operating Income									372	721	(256)
Total Depreciation and Amortization									354	469	222
Total Interest Charges									21	8	4
Total Income Taxes									186	146	(70)
Total Property Additions									970	2,762	1,820
Total Identifiable Assets	0				21,723				0	21,723
Subtotal SJI Utilities | Operating Segments | SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues									925,972	902,308	676,906
Total Operating Income									261,245	217,530	134,596
Total Depreciation and Amortization									160,032	123,430	96,424
Total Interest Charges									63,406	59,014	38,493
Total Income Taxes									47,975	40,729	23,257
Total Property Additions									464,709	464,454	345,696
Total Identifiable Assets	6,083,332				5,829,124				6,083,332	5,829,124
Wholesale Energy Operations | Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues									571,590	607,093	636,005
Total Operating Income									33,869	439	87,895
Total Depreciation and Amortization									69	89	105
Total Income Taxes									9,666	574	22,473
Total Property Additions									6	7	34
Total Identifiable Assets	195,882				195,576				195,882	195,576
Retail Gas and Other Operations | Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues									0	0	101,543
Total Operating Income									0	0	(8,721)
Total Depreciation and Amortization									0	0	279
Total Interest Charges									0	0	487
Total Income Taxes									0	0	(2,360)
Total Property Additions									0	4	495
Retail Electric Operations | Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues									34,005	81,193	176,945
Total Operating Income									(3,710)	(4,985)	(359)
Total Income Taxes									(524)	(935)	(101)
Total Identifiable Assets	14,797				30,351				14,797	30,351
Subtotal Energy Group | Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues									605,595	688,286	914,493
Total Operating Income									30,159	(4,546)	78,815
Total Depreciation and Amortization									69	89	384
Total Interest Charges									0	0	487
Total Income Taxes									9,142	(361)	20,012
Total Property Additions									6	11	529
Total Identifiable Assets	210,679				225,927				210,679	225,927
On-Site Energy Production | Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues									15,617	48,748	72,374
Total Operating Income									(5,602)	(4,248)	(88,230)
Total Depreciation and Amortization									5,647	4,591	23,123
Total Interest Charges									4,001	8,637	15,364
Total Income Taxes									(24,132)	(3,308)	(26,397)
Total Property Additions									85,280	229	2,686
Total Identifiable Assets	153,018				154,021				153,018	154,021
Appliance Service Operations | Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues									1,978	2,042	1,957
Total Operating Income									1,974	2,108	1,818
Total Depreciation and Amortization									0	0	0
Total Income Taxes									676	627	534
Total Property Additions									0	0	0
Subtotal Energy Services | Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues									17,595	50,790	74,331
Total Operating Income									(3,628)	(2,140)	(86,412)
Total Depreciation and Amortization									5,647	4,591	23,123
Total Income Taxes									(23,456)	(2,681)	(25,863)
Total Property Additions									85,280	229	2,686
Total Identifiable Assets	153,018				154,021				153,018	154,021
Midstream | Operating Segments
Segment Reporting Information [Line Items]
Total Operating Income									(467)	(154)	(292)
Total Interest Charges									2,513	2,262	1,966
Total Income Taxes									(217)	(135)	(190)
Total Property Additions									131	46	119
Total Identifiable Assets	92,208				83,517				92,208	83,517
Corporate & Services | Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues									56,690	44,511	51,000
Total Operating Income									(5,087)	(9,485)	(25,962)
Total Depreciation and Amortization									4,899	5,275	12,983
Total Interest Charges									55,916	57,716	54,107
Total Income Taxes									(10,780)	(16,491)	(16,655)
Total Property Additions									2,799	1,554	$ 1,826
Total Identifiable Assets	$ 373,467				$ 403,170				$ 373,467	$ 403,170