REVENUE - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue	$ 762,283	$ 475,956
Gas
Disaggregation of Revenue [Line Items]
Revenue	752,980	462,399
Electric
Disaggregation of Revenue [Line Items]
Revenue	1,529	5,711
Solar
Disaggregation of Revenue [Line Items]
Revenue	1,911	1,907
CHP
Disaggregation of Revenue [Line Items]
Revenue		3,502
Landfills
Disaggregation of Revenue [Line Items]
Revenue	425	1,579
Fuel Cells
Disaggregation of Revenue [Line Items]
Revenue	4,087
Other Product
Disaggregation of Revenue [Line Items]
Revenue	1,351	858
Residential
Disaggregation of Revenue [Line Items]
Revenue	273,005	237,242
Commercial and Industrial
Disaggregation of Revenue [Line Items]
Revenue	485,187	231,616
OSS and Capacity Release
Disaggregation of Revenue [Line Items]
Revenue	2,299	2,609
Other Customer
Disaggregation of Revenue [Line Items]
Revenue	1,792	4,489
SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	229,962	187,809
SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	229,962	187,809
SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	166,589	142,808
SJG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	60,313	41,835
SJG Utility Operations | OSS and Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	2,299	2,609
SJG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	761	557
ETG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	155,750	133,406
ETG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	155,750	133,406
ETG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	105,904	92,409
ETG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	49,654	37,148
ETG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	192	3,849
ELK Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		3,278
ELK Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		3,278
ELK Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		1,536
ELK Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		1,659
ELK Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		83
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	373,967	138,995
Wholesale Energy Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	373,967	138,995
Wholesale Energy Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	373,967	138,995
Retail Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	3,054	7,852
Retail Services | Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,607	6,994
Retail Services | Other Product | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,447	858
Retail Services | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	512	489
Retail Services | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,607	7,363
Retail Services | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	935
Renewables | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	6,423	6,988
Renewables | Solar | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	1,911	1,907
Renewables | CHP | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue		3,502
Renewables | Landfills | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	425	1,579
Renewables | Fuel Cells | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	4,087
Renewables | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	6,423	6,988
Corporate and Services | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(6,873)	(2,372)
Corporate and Services | Gas | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(6,699)	(1,089)
Corporate and Services | Electric | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(78)	(1,283)
Corporate and Services | Other Product | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(96)
Corporate and Services | Commercial and Industrial | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	(6,777)	$ (2,372)
Corporate and Services | Other Customer | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue	$ (96)