SEGMENTS OF BUSINESS (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) project	Mar. 31, 2020 USD ($) project	Dec. 31, 2020 USD ($)
Segment Reporting [Abstract]
Number of solar energy projects expected to be sold | project	3
Number of projects sold | project		1
Segment Reporting Information [Line Items]
Number of solar energy projects expected to be sold | project	3
Number of projects sold | project		1
Revenues	$ 674,300	$ 534,112
Operating Income (Loss)	196,828	165,762
Depreciation and Amortization	54,440	35,902
Interest Charges	31,459	32,536
Income Taxes	41,769	33,370
Property Additions	96,896	108,394
Identifiable Assets	6,714,508		$ 6,689,148
Discontinued Operations
Segment Reporting Information [Line Items]
Identifiable Assets	1,756		1,775
Operating Segments
Segment Reporting Information [Line Items]
Revenues	694,966	549,825
Interest Charges	33,434	34,768
Operating Segments | Midstream
Segment Reporting Information [Line Items]
Interest Charges	671	580
Income Taxes	(61)	(29)
Property Additions	4	45
Identifiable Assets	94,191		92,208
Operating Segments | Corporate and Services
Segment Reporting Information [Line Items]
Revenues	13,793	13,341
Operating Income (Loss)	178	172
Depreciation and Amortization	990	1,240
Interest Charges	12,843	17,887
Income Taxes	(3,182)	(5,615)
Property Additions	744	661
Identifiable Assets	281,016		318,095
Operating Segments | SJI Utilities | Subtotal SJI Utilities
Segment Reporting Information [Line Items]
Revenues	408,944	387,969
Operating Income (Loss)	176,862	159,242
Depreciation and Amortization	52,114	34,643
Interest Charges	18,418	14,699
Income Taxes	39,267	35,988
Property Additions	94,539	107,635
Identifiable Assets	6,138,078		6,083,332
Operating Segments | SJI Utilities | SJG Utility Operations
Segment Reporting Information [Line Items]
Revenues	251,399	240,694
Operating Income (Loss)	118,842	104,645
Depreciation and Amortization	29,484	25,059
Interest Charges	9,725	7,542
Income Taxes	27,114	25,231
Property Additions	53,409	57,970
Identifiable Assets	3,554,139		3,522,265
Operating Segments | SJI Utilities | ETG Utility Operations
Segment Reporting Information [Line Items]
Revenues	157,545	144,157
Operating Income (Loss)	58,020	54,062
Depreciation and Amortization	22,630	9,451
Interest Charges	8,693	7,145
Income Taxes	12,153	10,621
Property Additions	41,130	49,014
Identifiable Assets	2,583,939		2,561,067
Operating Segments | SJI Utilities | ELK Utility Operations
Segment Reporting Information [Line Items]
Revenues	0	3,118
Operating Income (Loss)	0	535
Depreciation and Amortization	0	133
Interest Charges	0	12
Income Taxes	0	136
Property Additions	0	651
Operating Segments | Energy Management | Subtotal Energy Management
Segment Reporting Information [Line Items]
Revenues	265,806	141,527
Operating Income (Loss)	17,088	6,644
Depreciation and Amortization	138	16
Income Taxes	5,084	1,977
Identifiable Assets	202,511		225,569
Operating Segments | Energy Management | Wholesale Energy Operations
Segment Reporting Information [Line Items]
Revenues	261,910	128,444
Operating Income (Loss)	17,308	7,412
Depreciation and Amortization	20	16
Income Taxes	4,912	2,006
Identifiable Assets	174,736		195,882
Operating Segments | Energy Management | Retail Services
Segment Reporting Information [Line Items]
Revenues	3,896	13,083
Operating Income (Loss)	(220)	(768)
Depreciation and Amortization	118	0
Income Taxes	172	(29)
Identifiable Assets	27,775		29,687
Operating Segments | Energy Production | Subtotal Energy Production
Segment Reporting Information [Line Items]
Revenues	6,423	6,988
Operating Income (Loss)	2,700	(296)
Depreciation and Amortization	1,198	3
Interest Charges	1,502	1,602
Income Taxes	661	1,049
Property Additions	1,609	53
Identifiable Assets	186,172		193,500
Operating Segments | Energy Production | Renewables
Segment Reporting Information [Line Items]
Revenues	6,423	6,988
Operating Income (Loss)	2,700	(296)
Depreciation and Amortization	1,198	3
Interest Charges	1,208	1,602
Income Taxes	464	1,049
Property Additions	773	53
Identifiable Assets	144,036		153,018
Operating Segments | Energy Production | Decarbonization Segment
Segment Reporting Information [Line Items]
Interest Charges	294	0
Income Taxes	197	0
Property Additions	836	0
Identifiable Assets	42,136		40,482
Intersegment
Segment Reporting Information [Line Items]
Revenues	(20,666)	(15,713)
Interest Charges	(1,975)	$ (2,232)
Identifiable Assets	$ (189,216)		$ (225,331)