SEGMENTS OF BUSINESS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 01, 2018
Segment Reporting Information [Line Items]
Property, Plant and Equipment												$ 1,089,342
Operating Revenues	$ 589,583	$ 302,480	$ 227,330	$ 521,945	$ 345,738	$ 227,127	$ 244,374	$ 425,829	$ 1,641,338	$ 1,243,068	$ 1,036,500
Operating Income (Loss)									100,745	8,843	194,177
Depreciation and Amortization									132,914	123,486	109,818
Interest Charges									90,296	54,019	31,449
Income Taxes	12,767	(16,649)	(31,972)	36,415	(16,498)	(24,765)	(5,544)	21,870	561	(24,937)	54,151
Property Additions									350,856	269,747	269,689
Identifiable Assets	5,956,577				3,865,086				5,956,577	3,865,086
SJG Utility Operations
Segment Reporting Information [Line Items]
Operating Revenues	180,369	56,371	76,801	234,459	170,434	66,755	83,251	196,814	548,000	517,254	461,055
Operating Income (Loss)									132,688	138,875	125,052
Interest Charges									28,011	24,705	17,875
Income Taxes	6,913	$ (2,818)	$ 482	$ 21,836	18,046	$ (3,688)	$ 1,431	$ 29,911	26,413	45,700	39,366
Identifiable Assets	3,118,236				2,865,974				3,118,236	2,865,974
Midstream
Segment Reporting Information [Line Items]
Identifiable Assets	72,333				63,112				72,333	63,112
ETG Utility Operations
Segment Reporting Information [Line Items]
Property, Plant and Equipment	1,077,000								1,077,000
ELK Utility Operations
Segment Reporting Information [Line Items]
Property, Plant and Equipment	12,300								12,300
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating Revenues									(75,392)	(68,410)	(57,593)
Interest Charges									(20,121)	(15,728)	(10,855)
Identifiable Assets	(301,735)				(661,363)				(301,735)	(661,363)
Discontinued Operations
Segment Reporting Information [Line Items]
Identifiable Assets	1,777				1,757				1,777	1,757
Energy Services | Energy Services
Segment Reporting Information [Line Items]
Identifiable Assets	195,329				583,925				195,329	583,925
Energy Services | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Identifiable Assets	195,329				582,587				195,329	582,587
Energy Services | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Identifiable Assets	0				1,338				0	1,338
Energy Group | Energy Group
Segment Reporting Information [Line Items]
Identifiable Assets	318,498				300,643				318,498	300,643
Energy Group | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Identifiable Assets	266,417				208,785				266,417	208,785
Energy Group | Energy Group | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Identifiable Assets	12,736				56,935				12,736	56,935
Energy Group | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Identifiable Assets	39,345				34,923				39,345	34,923
Midstream
Segment Reporting Information [Line Items]
Operating Income (Loss)									(292)	0	0
Interest Charges									1,966	985	0
Income Taxes									(190)	(41)	0
Corporate Services and Other Operations
Segment Reporting Information [Line Items]
Operating Revenues									51,000	45,024	35,147
Operating Income (Loss)									(25,962)	(10,932)	1,888
Depreciation and Amortization									12,983	4,303	1,400
Interest Charges									54,107	23,819	12,118
Income Taxes									(16,655)	(17,554)	44
Property Additions									1,826	2,233	636
Identifiable Assets	387,482				711,038				387,482	711,038
Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues									1,716,730	1,311,478	1,094,093
Interest Charges									110,417	69,747	42,304
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Property Additions									119	218	505
Operating Segments | Energy Group
Segment Reporting Information [Line Items]
Operating Revenues									914,493	643,195	495,618
Operating Income (Loss)									78,815	(35,663)	53,354
Depreciation and Amortization									384	448	821
Interest Charges									487	3,400	350
Income Taxes									20,012	(13,784)	20,862
Property Additions									529	903	1,649
Operating Segments | Energy Group | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Operating Revenues									636,005	352,613	220,707
Operating Income (Loss)									87,895	(36,815)	41,667
Depreciation and Amortization									105	125	484
Interest Charges									0	3,150	0
Income Taxes									22,473	(14,720)	15,882
Property Additions									34	14	7
Operating Segments | Energy Group | Retail Gas and Other Operations
Segment Reporting Information [Line Items]
Operating Revenues									101,543	111,048	92,371
Operating Income (Loss)									(8,721)	(2,468)	4,680
Depreciation and Amortization									279	323	337
Interest Charges									487	250	350
Income Taxes									(2,360)	(544)	2,118
Property Additions									495	889	1,642
Operating Segments | Energy Group | Retail Electric Operations
Segment Reporting Information [Line Items]
Operating Revenues									176,945	179,534	182,540
Operating Income (Loss)									(359)	3,620	7,007
Income Taxes									(101)	1,480	2,862
Operating Segments | Energy Services
Segment Reporting Information [Line Items]
Operating Revenues									74,331	106,005	102,273
Operating Income (Loss)									(86,412)	(83,437)	13,883
Depreciation and Amortization									23,123	47,081	43,696
Income Taxes									(25,863)	(39,258)	(6,121)
Property Additions									2,686	12,848	38,624
Operating Segments | Energy Services | On-Site Energy Production
Segment Reporting Information [Line Items]
Operating Revenues									72,374	99,517	94,375
Operating Income (Loss)									(88,230)	(83,654)	13,301
Depreciation and Amortization									23,123	46,928	43,395
Interest Charges									15,364	16,838	11,961
Income Taxes									(26,397)	(39,262)	(6,353)
Property Additions									2,686	12,588	38,193
Operating Segments | Energy Services | Appliance Service Operations
Segment Reporting Information [Line Items]
Operating Revenues									1,957	6,488	7,898
Operating Income (Loss)									1,818	217	582
Depreciation and Amortization									0	153	301
Income Taxes									534	4	232
Property Additions									0	260	431
Utility Operations
Segment Reporting Information [Line Items]
Identifiable Assets	5,282,893				2,865,974				5,282,893	2,865,974
Utility Operations | SJG Utility Operations
Segment Reporting Information [Line Items]
Identifiable Assets	3,118,236				2,865,974				3,118,236	2,865,974
Utility Operations | ETG Utility Operations
Segment Reporting Information [Line Items]
Identifiable Assets	2,148,175				0				2,148,175	0
Utility Operations | ELK Utility Operations
Segment Reporting Information [Line Items]
Identifiable Assets	$ 16,482				$ 0				16,482	0
Utility Operations | Operating Segments
Segment Reporting Information [Line Items]
Operating Revenues									676,906	517,254	461,055
Operating Income (Loss)									134,596	138,875	125,052
Depreciation and Amortization									96,424	71,654	63,901
Interest Income (Expense), Net									38,493	24,705	17,875
Income Taxes									23,257	45,700	39,366
Property Additions									345,696	253,545	228,275
Utility Operations | Operating Segments | SJG Utility Operations
Segment Reporting Information [Line Items]
Operating Revenues									548,000	517,254	461,055
Operating Income (Loss)									132,688	138,875	125,052
Depreciation and Amortization									82,622	71,654	63,901
Interest Income (Expense), Net									28,011	24,705	17,875
Income Taxes									26,413	45,700	39,366
Property Additions									253,617	253,545	228,275
Utility Operations | Operating Segments | ETG Utility Operations
Segment Reporting Information [Line Items]
Operating Revenues									125,604	0	0
Operating Income (Loss)									2,164	0	0
Depreciation and Amortization									13,580	0	0
Interest Income (Expense), Net									10,478	0	0
Income Taxes									(3,086)	0	0
Property Additions									90,259	0	0
Utility Operations | Operating Segments | ELK Utility Operations
Segment Reporting Information [Line Items]
Operating Revenues									3,302	0	0
Operating Income (Loss)									(256)	0	0
Depreciation and Amortization									222	0	0
Interest Income (Expense), Net									4	0	0
Income Taxes									(70)	0	0
Property Additions									$ 1,820	$ 0	$ 0