SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020 USD ($) project	Mar. 31, 2020 project	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) project	Sep. 30, 2019 USD ($) project	Dec. 31, 2019 USD ($) project
Segment Reporting [Abstract]
Number of solar energy projects expected to be sold | project	2			2		3
Number of projects sold | project		1		1	7
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 261,549		$ 261,203	$ 1,055,625	$ 1,165,435
Total Operating (Loss) Income	(8,494)		(19,025)	177,459	125,348
Total Depreciation and Amortization	37,516		33,547	110,724	98,582
Total Interest Charges	27,762		28,857	88,887	85,944
Total Income Taxes	(19,467)		(10,925)	13,725	9,378
Total Property Additions	179,657		124,338	396,933	341,259
Total Identifiable Assets	6,308,923			6,308,923		$ 6,365,340
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,790			1,790		1,766
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	273,304		274,547	1,096,603	1,206,390
Total Interest Charges	29,115		32,112	94,317	96,393
Operating Segments | Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	11,488		10,252	38,116	31,438
Total Operating (Loss) Income	(3,844)		(125)	(3,870)	(5,680)
Total Depreciation and Amortization	1,267		1,094	3,704	4,092
Total Interest Charges	12,490		14,433	43,332	44,496
Total Income Taxes	(3,187)		(3,299)	(10,982)	(11,715)
Total Property Additions	715		368	2,076	954
Total Identifiable Assets	249,943			249,943		403,170
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Total Interest Charges	677		573	1,837	1,672
Total Income Taxes	(58)		(18)	(174)	(83)
Total Property Additions	29		16	115	35
Total Identifiable Assets	90,459			90,459		83,517
Operating Segments | SJI Utilities | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	66,190		62,039	394,066	396,505
Total Operating (Loss) Income	(6,080)		(8,319)	108,297	100,241
Total Depreciation and Amortization	25,762		23,564	76,216	69,349
Total Interest Charges	8,271		7,840	23,832	23,584
Total Income Taxes	(3,293)		(3,747)	23,157	20,620
Total Property Additions	73,481		73,059	181,440	195,621
Total Identifiable Assets	3,383,449			3,383,449		3,348,555
Operating Segments | SJI Utilities | ETG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	37,503		30,619	239,503	215,647
Total Operating (Loss) Income	(3,163)		(5,112)	57,274	41,088
Total Depreciation and Amortization	10,391		7,461	30,370	20,932
Total Interest Charges	7,398		7,165	22,321	20,106
Total Income Taxes	(2,475)		(2,065)	7,869	4,028
Total Property Additions	45,452		50,426	149,578	142,388
Total Identifiable Assets	2,482,155			2,482,155		2,458,846
Operating Segments | SJI Utilities | ELK Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	285		892	4,792	5,210
Total Operating (Loss) Income	(374)		(197)	373	401
Total Depreciation and Amortization	52		158	354	383
Total Interest Charges	2		4	21	15
Total Income Taxes	(4)		(54)	186	100
Total Property Additions	112		468	971	2,096
Total Identifiable Assets	0			0		21,723
Operating Segments | SJI Utilities | Subtotal SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues	103,978		93,550	638,361	617,362
Total Operating (Loss) Income	(9,617)		(13,628)	165,944	141,730
Total Depreciation and Amortization	36,205		31,183	106,940	90,664
Total Interest Charges	15,671		15,009	46,174	43,705
Total Income Taxes	(5,772)		(5,866)	31,212	24,748
Total Property Additions	119,045		123,953	331,989	340,105
Total Identifiable Assets	5,865,604			5,865,604		5,829,124
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	143,338		135,856	373,308	452,346
Total Operating (Loss) Income	2,768		(8,371)	16,272	(13,263)
Total Depreciation and Amortization	14		22	44	70
Total Income Taxes	858		(2,130)	4,583	(3,088)
Total Property Additions	2		1	2	1
Total Identifiable Assets	126,973			126,973		195,576
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	7,692		22,395	29,874	65,617
Total Operating (Loss) Income	(842)		(3)	(2,512)	(3,972)
Total Income Taxes	(233)		4	(478)	(814)
Total Identifiable Assets	21,176			21,176		30,351
Operating Segments | Energy Group | Subtotal Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues	151,030		158,251	403,182	517,963
Total Operating (Loss) Income	1,926		(8,374)	13,760	(17,235)
Total Depreciation and Amortization	14		22	44	70
Total Income Taxes	625		(2,126)	4,105	(3,902)
Total Property Additions	2		1	2	1
Total Identifiable Assets	148,149			148,149		225,927
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	6,317		11,980	15,459	38,098
Total Operating (Loss) Income	2,455		2,634	143	5,049
Total Depreciation and Amortization	30		1,248	36	3,756
Total Interest Charges	277		2,097	2,974	6,520
Total Income Taxes	(11,191)		242	(10,897)	(115)
Total Property Additions	59,866		0	62,751	164
Total Identifiable Assets	113,050			113,050		154,021
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	491		514	1,485	1,529
Total Operating (Loss) Income	586		468	1,482	1,484
Total Income Taxes	116		142	461	445
Operating Segments | Energy Services | Subtotal Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues	6,808		12,494	16,944	39,627
Total Operating (Loss) Income	3,041		3,102	1,625	6,533
Total Depreciation and Amortization	30		1,248	36	3,756
Total Income Taxes	(11,075)		384	(10,436)	330
Total Property Additions	59,866		0	62,751	164
Total Identifiable Assets	113,050			113,050		154,021
Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues	(11,755)		(13,344)	(40,978)	(40,955)
Total Interest Charges	(1,353)		$ (3,255)	(5,430)	$ (10,449)
Total Identifiable Assets	$ (160,072)			$ (160,072)		$ (332,185)