SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) category	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Total Operating Revenues	$ 302,480	$ 227,127	$ 1,051,755	$ 897,330
Total Operating (Loss) Income	(38,592)	(54,077)	10,746	11,402
Total Depreciation and Amortization	30,896	30,084	101,450	89,316
Total Interest Charges	26,534	10,567	60,067	38,291
Total Income Taxes	(16,649)	(24,765)	(12,206)	(8,439)
Total Property Additions	82,711	61,121	211,979	201,574
Total Identifiable Assets	5,914,522		5,914,522		$ 3,865,086
Discontinued Operations
Segment Reporting Information [Line Items]
Total Identifiable Assets	1,781		$ 1,781		1,757
Energy Group and Energy Services
Segment Reporting Information [Line Items]
Number of operating categories | category			2
Operating Segments
Segment Reporting Information [Line Items]
Total Operating Revenues	315,638	242,039	$ 1,105,471	947,238
Total Interest Charges	32,046	14,934	75,712	50,104
Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	86,258	66,755	397,518	346,820
Total Operating (Loss) Income	(27,153)	(4,049)	74,091	85,911
Total Depreciation and Amortization	26,924	17,751	67,513	52,559
Total Interest Charges	11,946	6,437	25,673	18,392
Total Income Taxes	(9,827)	(3,688)	12,491	27,654
Total Property Additions	82,108	59,179	197,493	187,587
Total Identifiable Assets	5,025,920		5,025,920		2,865,974
Operating Segments | Energy Group:
Segment Reporting Information [Line Items]
Total Operating Revenues	204,428	134,213	612,057	488,353
Total Operating (Loss) Income	(10,845)	(12,264)	51,127	(42,847)
Total Depreciation and Amortization	111	111	316	339
Total Interest Charges	141	(107)	392	3,235
Total Income Taxes	(2,735)	(4,647)	13,103	(17,383)
Total Property Additions	186	204	527	637
Total Identifiable Assets	272,524		272,524		300,643
Operating Segments | Energy Group: | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	134,867	70,741	392,430	274,667
Total Operating (Loss) Income	(11,992)	(11,346)	53,193	(41,163)
Total Depreciation and Amortization	36	31	88	92
Total Interest Charges	0	(162)	0	3,031
Total Income Taxes	(3,036)	(4,281)	13,613	(16,984)
Total Property Additions	0	0	32	5
Total Identifiable Assets	187,258		187,258		208,785
Operating Segments | Energy Group: | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	18,292	18,156	81,661	76,793
Total Operating (Loss) Income	590	(574)	(3,509)	(3,801)
Total Depreciation and Amortization	75	80	228	247
Total Interest Charges	141	55	392	204
Total Income Taxes	144	(225)	(916)	(1,265)
Total Property Additions	186	204	495	632
Total Identifiable Assets	43,899		43,899		56,935
Operating Segments | Energy Group: | Retail Electric Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	51,269	45,316	137,966	136,893
Total Operating (Loss) Income	557	(344)	1,443	2,117
Total Income Taxes	157	(141)	406	866
Total Identifiable Assets	41,367		41,367		34,923
Operating Segments | Energy Services:
Segment Reporting Information [Line Items]
Total Operating Revenues	15,826	31,494	62,688	79,879
Total Operating (Loss) Income	2,644	(38,743)	(97,400)	(35,614)
Total Depreciation and Amortization	1,210	11,774	21,805	35,151
Total Income Taxes	(160)	(16,490)	(27,569)	(19,321)
Total Property Additions	696	1,633	2,379	12,159
Total Identifiable Assets	477,601		477,601		583,925
Operating Segments | Energy Services: | On-Site Energy Production
Segment Reporting Information [Line Items]
Total Operating Revenues	15,317	29,942	61,208	74,689
Total Operating (Loss) Income	2,966	(38,351)	(98,023)	(35,216)
Total Depreciation and Amortization	1,210	11,731	21,805	34,998
Total Interest Charges	4,115	3,549	12,060	13,240
Total Income Taxes	(331)	(16,270)	(27,977)	(19,120)
Total Property Additions	696	1,633	2,379	11,899
Total Identifiable Assets	477,573		477,573		582,587
Operating Segments | Energy Services: | Appliance Service Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	509	1,552	1,480	5,190
Total Operating (Loss) Income	(322)	(392)	623	(398)
Total Depreciation and Amortization	0	43	0	153
Total Income Taxes	171	(220)	408	(201)
Total Property Additions	0	0	0	260
Total Identifiable Assets	28		28		1,338
Corporate and Services
Segment Reporting Information [Line Items]
Total Operating Revenues	9,126	9,577	33,208	32,186
Total Operating (Loss) Income	(3,238)	979	(17,072)	3,952
Total Depreciation and Amortization	2,651	448	11,816	1,267
Total Interest Charges	15,303	4,895	36,141	14,442
Total Income Taxes	(3,827)	60	(10,171)	696
Total Property Additions	0	63	11,549	991
Total Identifiable Assets	621,152		621,152		711,038
Intersegment Sales
Segment Reporting Information [Line Items]
Total Operating Revenues	(13,158)	(14,912)	(53,716)	(49,908)
Total Interest Charges	(5,512)	(4,367)	(15,645)	(11,813)
Total Identifiable Assets	(555,209)		(555,209)		(661,363)
Elizabethtown Gas | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	29,117	0	29,117	0
Total Operating (Loss) Income	(19,808)	0	(19,808)	0
Total Depreciation and Amortization	6,403	0	6,403	0
Total Interest Charges	4,835	0	4,835	0
Total Income Taxes	(6,866)	0	(6,866)	0
Total Property Additions	18,637	0	18,637	0
Total Identifiable Assets	2,044,378		2,044,378		0
Elkton Gas | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	770	0	770	0
Total Operating (Loss) Income	(518)	0	(518)	0
Total Depreciation and Amortization	94	0	94	0
Total Interest Charges	3	0	3	0
Total Income Taxes	(143)	0	(143)	0
Total Property Additions	129	0	129	0
Total Identifiable Assets	15,158		15,158		0
SJI Midstream, LLC
Segment Reporting Information [Line Items]
Total Identifiable Assets	70,753		70,753		63,112
SJI Midstream, LLC | Operating Segments
Segment Reporting Information [Line Items]
Total Interest Charges	541	160	1,446	795
Total Income Taxes	(100)	0	(60)	(85)
Total Property Additions	(279)	42	31	200
SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	56,371	66,755	367,631	346,820
Total Operating (Loss) Income	(6,827)	(4,049)	94,417	85,911
Total Income Taxes	(2,818)	(3,688)	19,500	27,654
Total Identifiable Assets	2,966,384		2,966,384		2,865,974
SJG Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Total Operating Revenues	56,371	66,755	367,631	346,820
Total Operating (Loss) Income	(6,827)	(4,049)	94,417	85,911
Total Depreciation and Amortization	20,427	17,751	61,016	52,559
Total Interest Charges	7,108	6,437	20,835	18,392
Total Income Taxes	(2,818)	(3,688)	19,500	27,654
Total Property Additions	63,342	$ 59,179	178,727	$ 187,587
Total Identifiable Assets	$ 2,966,384		$ 2,966,384		$ 2,865,974