SEGMENTS OF BUSINESS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 261,203	$ 302,480	$ 1,165,435	$ 1,051,755
Total Operating (Loss) Income	(19,025)	(38,592)	125,348	10,746
Total Depreciation and Amortization	33,547	30,896	98,582	101,450
Total Interest Charges	28,857	26,534	85,944	60,067
Total Income Taxes	(10,925)	(16,649)	9,378	(12,206)
Total Property Additions	124,338	82,711	341,259	211,979
Total Identifiable Assets	6,130,053		6,130,053		$ 5,956,577
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,778		1,778		1,777
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	274,547	315,638	1,206,390	1,105,471
Total Interest Charges	32,112	32,046	96,393	75,712
Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	93,550	86,258	617,362	397,518
Total Operating (Loss) Income	(13,628)	(27,153)	141,730	74,091
Total Depreciation and Amortization	31,183	26,924	90,664	67,513
Total Interest Charges	15,009	11,946	43,705	25,673
Total Income Taxes	(5,866)	(9,827)	24,748	12,491
Total Property Additions	123,953	82,108	340,105	197,493
Total Identifiable Assets	5,609,248		5,609,248		5,282,893
Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Total Operating Revenues	158,251	204,428	517,963	612,057
Total Operating (Loss) Income	(8,374)	(10,845)	(17,235)	51,127
Total Depreciation and Amortization	22	111	70	316
Total Interest Charges	0	141	0	392
Total Income Taxes	(2,126)	(2,735)	(3,902)	13,103
Total Property Additions	1	186	1	527
Total Identifiable Assets	202,944		202,944		318,498
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	135,856	134,867	452,346	392,430
Total Operating (Loss) Income	(8,371)	(11,992)	(13,263)	53,193
Total Depreciation and Amortization	22	36	70	88
Total Income Taxes	(2,130)	(3,036)	(3,088)	13,613
Total Property Additions	1	0	1	32
Total Identifiable Assets	173,252		173,252		266,417
Operating Segments | Energy Group | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	0	18,292	0	81,661
Total Operating (Loss) Income	0	590	0	(3,509)
Total Depreciation and Amortization	0	75	0	228
Total Interest Charges	0	141	0	392
Total Income Taxes	0	144	0	(916)
Total Property Additions	0	186	0	495
Total Identifiable Assets	120		120		12,736
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	22,395	51,269	65,617	137,966
Total Operating (Loss) Income	(3)	557	(3,972)	1,443
Total Income Taxes	4	157	(814)	406
Total Identifiable Assets	29,572		29,572		39,345
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Total Operating Revenues	12,494	15,826	39,627	62,688
Total Operating (Loss) Income	3,102	2,644	6,533	(97,400)
Total Depreciation and Amortization	1,248	1,210	3,756	21,805
Total Income Taxes	384	(160)	330	(27,569)
Total Property Additions	0	696	164	2,379
Total Identifiable Assets	163,716		163,716		195,329
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	11,980	15,317	38,098	61,208
Total Operating (Loss) Income	2,634	2,966	5,049	(98,023)
Total Depreciation and Amortization	1,248	1,210	3,756	21,805
Total Interest Charges	2,097	4,115	6,520	12,060
Total Income Taxes	242	(331)	(115)	(27,977)
Total Property Additions	0	696	164	2,379
Total Identifiable Assets	163,716		163,716		195,329
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	514	509	1,529	1,480
Total Operating (Loss) Income	468	(322)	1,484	623
Total Depreciation and Amortization	0	0	0	0
Total Income Taxes	142	171	445	408
Total Identifiable Assets	0		0		0
Corporate Services and Intersegment
Segment Reporting Information [Line Items]
Total Operating Revenues	10,252	9,126	31,438	33,208
Total Operating (Loss) Income	(125)	(3,238)	(5,680)	(17,072)
Total Depreciation and Amortization	1,094	2,651	4,092	11,816
Total Interest Charges	14,433	15,303	44,496	36,141
Total Income Taxes	(3,299)	(3,827)	(11,715)	(10,171)
Total Property Additions	368	0	954	11,549
Total Identifiable Assets	394,897		394,897		387,482
Intersegment sales
Segment Reporting Information [Line Items]
Total Operating Revenues	(13,344)	(13,158)	(40,955)	(53,716)
Total Interest Charges	(3,255)	(5,512)	(10,449)	(15,645)
Total Identifiable Assets	(323,629)		(323,629)		(301,735)
ETG Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	30,619	29,117	215,647	29,117
Total Operating (Loss) Income	(5,112)	(19,808)	41,088	(19,808)
Total Depreciation and Amortization	7,461	6,403	20,932	6,403
Total Interest Charges	7,165	4,835	20,106	4,835
Total Income Taxes	(2,065)	(6,866)	4,028	(6,866)
Total Property Additions	50,426	18,637	142,388	18,637
Total Identifiable Assets	2,335,164		2,335,164		2,148,175
ELK Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	892	770	5,210	770
Total Operating (Loss) Income	(197)	(518)	401	(518)
Total Depreciation and Amortization	158	94	383	94
Total Interest Charges	4	3	15	3
Total Income Taxes	(54)	(143)	100	(143)
Total Property Additions	468	129	2,096	129
Total Identifiable Assets	19,808		19,808		16,482
SJG
Segment Reporting Information [Line Items]
Total Operating Revenues		56,371	396,505	367,631
Total Operating (Loss) Income	(8,319)	(6,827)	100,241	94,417
Total Income Taxes	(3,747)	(2,818)	20,620	19,500
Total Identifiable Assets	3,254,276		3,254,276		3,118,236
SJG | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	62,039	56,371	396,505	367,631
Total Operating (Loss) Income	(8,319)	(6,827)	100,241	94,417
Total Depreciation and Amortization	23,564	20,427	69,349	61,016
Total Interest Charges	7,840	7,108	23,584	20,835
Total Income Taxes	(3,747)	(2,818)	20,620	19,500
Total Property Additions	73,059	63,342	195,621	178,727
Total Identifiable Assets	3,254,276		3,254,276		3,118,236
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	81,099		81,099		$ 72,333
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	573	541	1,672	1,446
Total Income Taxes	(18)	(100)	(83)	(60)
Total Property Additions	$ 16	$ (279)	$ 35	$ 31