SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($) project	Mar. 31, 2020 project	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($) project	Dec. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of legacy solar energy projects | project			3			3
Number of projects sold | project				1
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 365,631		$ 261,549		$ 1,351,759	$ 1,055,625
Total Operating (Loss) Income	(8,973)		(8,494)		199,464	177,459
Total Depreciation and Amortization	48,089		37,516		151,709	110,724
Total Interest Charges	31,468		27,762		94,112	88,887
Total Income Taxes	(11,727)		(19,467)		25,340	13,725
Total Property Additions	139,284		179,657		383,503	396,933
Total Identifiable Assets	7,024,011				7,024,011		$ 6,689,148
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,811				1,811		1,775
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	380,721		273,304		1,400,515	1,096,603
Total Interest Charges	33,223		29,115		99,730	94,317
Operating Segments | Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	13,577		10,835		39,416	36,740
Total Operating (Loss) Income	315		(2,527)		(626)	(2,662)
Total Depreciation and Amortization	1,858		1,267		4,271	3,704
Total Interest Charges	13,406		12,490		39,744	43,332
Total Income Taxes	(3,568)		(3,344)		(10,129)	(11,167)
Total Property Additions	747		715		3,105	2,076
Total Identifiable Assets	320,268				320,268		318,095
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Total Interest Charges	322		677		1,627	1,837
Total Income Taxes	(107)		(58)		(197)	(174)
Total Property Additions	11		29		19	115
Total Identifiable Assets	8,795				8,795		92,208
Operating Segments | SJI Utilities | Subtotal SJI Utilities
Segment Reporting Information [Line Items]
Total Operating Revenues	107,030		103,978		660,670	638,361
Total Operating (Loss) Income	3,673		(9,617)		199,871	165,944
Total Depreciation and Amortization	44,917		36,205		143,443	106,940
Total Interest Charges	18,095		15,671		53,880	46,174
Total Income Taxes	(2,328)		(5,772)		38,003	31,212
Total Property Additions	123,911		119,045		345,901	331,989
Total Identifiable Assets	6,354,888				6,354,888		6,083,332
Operating Segments | SJI Utilities | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	69,958		66,190		412,572	394,066
Total Operating (Loss) Income	(1,961)		(6,080)		133,350	108,297
Total Depreciation and Amortization	30,252		25,762		89,786	76,216
Total Interest Charges	9,444		8,271		28,782	23,832
Total Income Taxes	(2,460)		(3,293)		26,683	23,157
Total Property Additions	64,988		73,481		186,948	181,440
Total Identifiable Assets	3,637,046				3,637,046		3,522,265
Operating Segments | SJI Utilities | ETG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	37,072		37,503		248,098	239,503
Total Operating (Loss) Income	5,634		(3,163)		66,521	57,274
Total Depreciation and Amortization	14,665		10,391		53,657	30,370
Total Interest Charges	8,651		7,398		25,098	22,321
Total Income Taxes	132		(2,475)		11,320	7,869
Total Property Additions	58,923		45,452		158,953	149,578
Total Identifiable Assets	2,717,842				2,717,842		2,561,067
Operating Segments | SJI Utilities | ELK Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	0		285		0	4,792
Total Operating (Loss) Income	0		(374)		0	373
Total Depreciation and Amortization	0		52		0	354
Total Interest Charges	0		2		0	21
Total Income Taxes	0		(4)		0	186
Total Property Additions	0		112		0	971
Operating Segments | Energy Management | Subtotal Energy Management
Segment Reporting Information [Line Items]
Total Operating Revenues	252,762		152,174		682,449	406,043
Total Operating (Loss) Income	(14,685)		1,195		(4,630)	14,034
Total Depreciation and Amortization	142		14		422	44
Total Interest Charges	36		0		135	0
Total Income Taxes	(3,795)		898		(365)	4,751
Total Property Additions		$ 4	2		6	2
Total Identifiable Assets	299,536				299,536		225,569
Operating Segments | Energy Management | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	247,402		143,338		669,067	373,308
Total Operating (Loss) Income	(15,534)		2,768		(5,393)	16,272
Total Depreciation and Amortization	25		14		69	44
Total Income Taxes	(4,043)		858		(907)	4,583
Total Property Additions	4		2		6	2
Total Identifiable Assets	272,955				272,955		195,882
Operating Segments | Energy Management | Retail Services
Segment Reporting Information [Line Items]
Total Operating Revenues	5,360		8,836		13,382	32,735
Total Operating (Loss) Income	849		(1,573)		763	(2,238)
Total Depreciation and Amortization	117		0		353	0
Total Interest Charges	36		0		135	0
Total Income Taxes	248		40		542	168
Total Identifiable Assets	26,581				26,581		29,687
Operating Segments | Energy Production | Decarbonization
Segment Reporting Information [Line Items]
Total Operating (Loss) Income	(63)		0		(71)	0
Total Interest Charges	286		0		853	0
Total Income Taxes	205		0		329	0
Total Property Additions	8,140		0		10,014	0
Total Identifiable Assets	89,910				89,910		40,482
Operating Segments | Energy Production | Subtotal Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	7,352		6,317		17,980	15,459
Total Operating (Loss) Income	1,724		2,455		4,849	143
Total Depreciation and Amortization	1,172		30		3,573	36
Total Interest Charges	1,364		277		4,344	2,974
Total Income Taxes	(1,929)		(11,191)		(1,972)	(10,897)
Total Property Additions	14,611		59,866		34,472	62,751
Total Identifiable Assets	254,396				254,396		193,500
Operating Segments | Energy Production | Renewables
Segment Reporting Information [Line Items]
Total Operating Revenues	7,352		6,317		17,980	15,459
Total Operating (Loss) Income	1,787		2,455		4,920	143
Total Depreciation and Amortization	1,172		30		3,573	36
Total Interest Charges	1,078		277		3,491	2,974
Total Income Taxes	(2,134)		(11,191)		(2,301)	(10,897)
Total Property Additions	6,471		59,866		24,458	62,751
Total Identifiable Assets	164,486				164,486		153,018
Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues	(15,090)		(11,755)		(48,756)	(40,978)
Total Interest Charges	(1,755)		$ (1,353)		(5,618)	$ (5,430)
Total Identifiable Assets	$ (215,683)				$ (215,683)		$ (225,331)