REVENUES - Disaggregated Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Utility	$ 1,560,083	$ 1,552,547	$ 1,510,174
Gas
Disaggregation of Revenue [Line Items]
Utility	1,522,816	1,452,183	1,325,809
Electric
Disaggregation of Revenue [Line Items]
Utility	17,936	48,964	116,341
Solar
Disaggregation of Revenue [Line Items]
Utility	8,426	15,111	29,137
CHP
Disaggregation of Revenue [Line Items]
Utility	3,502	27,993	30,473
Landfills
Disaggregation of Revenue [Line Items]
Utility	3,689	5,644	6,457
Other
Disaggregation of Revenue [Line Items]
Utility	3,714	2,652	1,957
Residential
Disaggregation of Revenue [Line Items]
Utility	576,489	593,541	445,171
Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	972,041	937,191	1,047,754
OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Utility	7,673	8,951	11,536
Other
Disaggregation of Revenue [Line Items]
Utility	3,880	12,864	5,713
SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	503,132	485,012	475,497
SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	503,132	485,012	475,497
SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	349,111	356,646	329,207
SJG Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	144,300	116,959	132,055
SJG Utility Operations | OSS & Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	7,673	8,951	11,536
SJG Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,048	2,456	2,699
ETG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	339,150	331,027	128,647
ETG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	339,150	331,027	128,647
ETG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	223,221	217,195	82,763
ETG Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	114,300	103,590	42,935
ETG Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,629	10,242	2,949
ELK Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	4,926	7,857	3,362
ELK Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	4,926	7,857	3,362
ELK Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,179	3,494	1,482
ELK Utility Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,544	4,197	1,815
ELK Utility Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	203	166	65
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	683,152	633,720	652,833
Wholesale Energy Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	683,152	633,720	652,833
Wholesale Energy Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	683,152	633,720	652,833
Retail Gas Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			75,651
Retail Gas Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			75,651
Retail Gas Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			75,651
Retail Electric Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	19,907	56,899	124,245
Retail Electric Operations | Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	19,907	56,899	124,245
Retail Electric Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility		14,164	29,762
Retail Electric Operations | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	19,907	42,735	94,483
On-Site Energy Production | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	15,617	48,748	72,374
On-Site Energy Production | Solar | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	8,426	15,111	35,444
On-Site Energy Production | CHP | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	3,502	27,993	30,473
On-Site Energy Production | Landfills | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	3,689	5,644	6,457
On-Site Energy Production | Commercial & Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	15,617	48,748	72,374
Appliance Service Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,978	2,042	1,957
Appliance Service Operations | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,978	2,042	1,957
Appliance Service Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,978	2,042	1,957
Corporate & Services | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(7,779)	(12,758)	(24,392)
Corporate & Services | Gas | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(7,544)	(5,433)	(10,181)
Corporate & Services | Electric | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(1,971)	(7,935)	(7,904)
Corporate & Services | Solar | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility			(6,307)
Corporate & Services | Other | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	1,736	610
Corporate & Services | Commercial & Industrial | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	$ (7,779)	$ (12,758)	$ (24,392)