REVENUE - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 519,083	$ 315,148	$ 1,678,048	$ 1,077,434
Gas
Disaggregation of Revenue [Line Items]
Revenue	507,114	303,198	1,649,258	1,043,638
Electric
Disaggregation of Revenue [Line Items]
Revenue	2,807	4,490	6,244	15,476
Solar
Disaggregation of Revenue [Line Items]
Revenue	1,961	5,429	4,776	8,725
CHP
Disaggregation of Revenue [Line Items]
Revenue		0	0	3,502
Landfills
Disaggregation of Revenue [Line Items]
Revenue	328	888	1,037	3,232
Fuel Cells
Disaggregation of Revenue [Line Items]
Revenue	5,063		12,167
Fuel Cells | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			12,167
Other
Disaggregation of Revenue [Line Items]
Revenue	1,810	1,143	4,566	2,861
Residential
Disaggregation of Revenue [Line Items]
Revenue	52,556	62,442	408,433	401,274
Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	461,657	250,252	1,256,644	664,471
OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue	2,504	1,859	7,053	5,931
Other
Disaggregation of Revenue [Line Items]
Revenue	2,366	595	5,918	5,758
Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue			3,469
SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	63,785	67,765	379,401	352,127
SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	63,785	67,765	379,401	352,127
SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	33,075	40,900	250,697	244,114
SJG Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	27,538	24,625	119,565	100,689
SJG Utility Operations | OSS & Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,504	1,859	7,053	5,931
SJG Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	668	381	2,086	1,393
ETG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	37,638	38,163	246,466	232,410
ETG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	37,638	38,163	246,466	232,410
ETG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	18,993	20,974	156,254	153,496
ETG Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	18,269	16,998	89,464	74,752
ETG Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	376	191	748	4,162
ELK Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		285		4,926
ELK Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		285		4,926
ELK Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		77		2,179
ELK Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		185		2,544
ELK Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		23		203
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	406,985	197,580	1,032,181	456,427
Wholesale Energy Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	406,985	197,580	1,032,181	456,427
Wholesale Energy Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	406,985	197,580	1,032,181	456,427
Retail Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	4,836	5,957	11,360	20,323
Retail Services | Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,846	4,814	6,409	17,462
Retail Services | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,990	1,143	4,951	2,861
Retail Services | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	488	491	1,482	1,485
Retail Services | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,846	5,466	6,409	18,838
Retail Services | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,502
Renewables | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	7,352	6,317	17,980	15,459
Renewables | Solar | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,961	5,429	4,776	8,725
Renewables | CHP | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		0	0	3,502
Renewables | Landfills | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	328	888	1,037	3,232
Renewables | Fuel Cells | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	5,063
Renewables | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	7,352	6,317	17,980	15,459
Corporate and Services | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(1,513)	(919)	(9,340)	(4,238)
Corporate and Services | Gas | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(1,294)	(595)	(8,790)	(2,252)
Corporate and Services | Electric | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(39)	(324)	(165)	(1,986)
Corporate and Services | Other | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(180)	0	(385)	0
Corporate and Services | Residential | Intersegment
Disaggregation of Revenue [Line Items]
Revenue			0
Corporate and Services | Commercial & Industrial | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(1,333)	$ (919)	(8,955)	$ (4,238)
Corporate and Services | Other | Intersegment
Disaggregation of Revenue [Line Items]
Revenue	$ (180)		$ (385)