SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($) project	Mar. 31, 2020 project	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) project	Jun. 30, 2019 USD ($) project	Dec. 31, 2019 USD ($) project
Segment Reporting [Abstract]
Number of solar energy sites expected to be sold | project	2			2		3
Number of projects closed | project		1		1	7
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 259,964		$ 266,934	$ 794,076	$ 904,232
Total Operating Income	20,191		8,866	185,953	143,419
Total Depreciation and Amortization	37,306		33,044	73,208	65,035
Total Interest Charges	28,589		28,434	61,125	57,087
Total Income Taxes	(178)		(4,646)	33,192	20,303
Total Property Additions	108,882		122,327	217,276	216,921
Total Identifiable Assets	6,216,954			6,216,954		$ 6,365,340
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,737			1,737		1,766
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	273,474		282,167	823,299	931,843
Total Interest Charges	30,434		31,873	65,202	64,281
Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	146,414		108,066	534,383	523,812
Total Operating Income	16,319		12,234	175,561	155,358
Total Depreciation and Amortization	36,092		30,009	70,735	59,481
Total Interest Charges	15,804		14,521	30,503	28,696
Total Income Taxes	996		(147)	36,984	30,615
Total Property Additions	105,309		122,178	212,944	216,152
Total Identifiable Assets	5,820,960			5,820,960		5,829,124
Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues	111,483		147,014	252,152	359,712
Total Operating Income	5,643		(4,699)	11,834	(8,861)
Total Depreciation and Amortization	14		25	30	48
Total Income Taxes	1,677		(1,050)	3,480	(1,776)
Total Identifiable Assets	172,414			172,414		225,927
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	101,526		126,483	229,970	316,490
Total Operating Income	6,092		(2,386)	13,504	(4,892)
Total Depreciation and Amortization	14		25	30	48
Total Income Taxes	1,719		(481)	3,725	(958)
Total Identifiable Assets	148,869			148,869		195,576
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	9,957		20,531	22,182	43,222
Total Operating Income	(449)		(2,313)	(1,670)	(3,969)
Total Income Taxes	(42)		(569)	(245)	(818)
Total Identifiable Assets	23,545			23,545		30,351
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues	2,659		15,272	10,136	27,133
Total Operating Income	(1,559)		1,764	(1,416)	2,477
Total Depreciation and Amortization	3		1,256	6	2,508
Total Income Taxes	(577)		257	639	(54)
Total Property Additions	2,832		141	2,885	164
Total Identifiable Assets	48,985			48,985		154,021
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	2,154		14,788	9,142	26,118
Total Operating Income	(2,016)		1,343	(2,312)	1,461
Total Depreciation and Amortization	3		1,256	6	2,508
Total Interest Charges	1,095		2,138	2,697	4,423
Total Income Taxes	(755)		121	294	(357)
Total Property Additions	2,832		141	2,885	164
Total Identifiable Assets	48,924			48,924		154,021
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	505		484	994	1,015
Total Operating Income	457		421	896	1,016
Total Income Taxes	178		136	345	303
Total Identifiable Assets	61			61		0
Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	12,918		11,815	26,628	21,186
Total Operating Income	(212)		(433)	(26)	(5,555)
Total Depreciation and Amortization	1,197		1,754	2,437	2,998
Total Interest Charges	12,955		14,659	30,842	30,063
Total Income Taxes	(2,187)		(3,673)	(7,795)	(8,417)
Total Property Additions	700		1	1,361	586
Total Identifiable Assets	182,348			182,348		403,170
Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues	(13,510)		(15,233)	(29,223)	(27,611)
Total Interest Charges	(1,845)		(3,439)	(4,077)	(7,194)
Total Identifiable Assets	(97,568)			(97,568)		(332,185)
ETG Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	57,843		44,854	202,000	185,028
Total Operating Income	6,375		2,051	60,437	46,200
Total Depreciation and Amortization	10,528		6,813	19,979	13,471
Total Interest Charges	7,778		6,620	14,923	12,941
Total Income Taxes	(277)		(809)	10,344	6,093
Total Property Additions	55,112		53,938	104,126	91,962
Total Identifiable Assets	2,451,923			2,451,923		2,458,846
ELK Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	1,389		944	4,507	4,318
Total Operating Income	212		(32)	747	598
Total Depreciation and Amortization	169		113	302	225
Total Interest Charges	7		5	19	11
Total Income Taxes	54		(9)	190	154
Total Property Additions	208		983	859	1,628
Total Identifiable Assets	21,137			21,137		21,723
SJG
Segment Reporting Information [Line Items]
Total Operating Revenues	87,182		62,268	327,876	334,466
Total Operating Income	9,732		10,215	114,377	108,560
Total Income Taxes	1,219		671	26,450	24,368
Total Identifiable Assets	3,347,900			3,347,900		3,348,555
SJG | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	87,182		62,268	327,876	334,466
Total Operating Income	9,732		10,215	114,377	108,560
Total Depreciation and Amortization	25,395		23,083	50,454	45,785
Total Interest Charges	8,019		7,896	15,561	15,744
Total Income Taxes	1,219		671	26,450	24,368
Total Property Additions	49,989		67,257	107,959	122,562
Total Identifiable Assets	3,347,900			3,347,900		3,348,555
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	88,078			88,078		$ 83,517
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	580		555	1,160	1,099
Total Income Taxes	(87)		(33)	(116)	(65)
Total Property Additions	$ 41		$ 7	$ 86	$ 19