REVENUE - Disaggregated Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Utility	$ 1,552,547	$ 1,510,174
Residential
Disaggregation of Revenue [Line Items]
Utility	593,541	445,171
Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	937,191	1,047,754
OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Utility	8,951	11,536
Other
Disaggregation of Revenue [Line Items]
Utility	12,864	5,713
Gas
Disaggregation of Revenue [Line Items]
Utility	1,452,183	1,325,809
Electric
Disaggregation of Revenue [Line Items]
Utility	48,964	116,341
Solar
Disaggregation of Revenue [Line Items]
Utility	15,111	29,137
CHP
Disaggregation of Revenue [Line Items]
Utility	27,993	30,473
Landfills
Disaggregation of Revenue [Line Items]
Utility	5,644	6,457
Other
Disaggregation of Revenue [Line Items]
Utility	2,652	1,957
Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(12,758)	(24,392)
Corporate Services and Intersegment | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	(12,758)	(24,392)
Corporate Services and Intersegment | Gas
Disaggregation of Revenue [Line Items]
Utility	(5,433)	(10,181)
Corporate Services and Intersegment | Electric
Disaggregation of Revenue [Line Items]
Utility	(7,935)	(7,904)
Corporate Services and Intersegment | Solar
Disaggregation of Revenue [Line Items]
Utility		(6,307)
Corporate Services and Intersegment | Other
Disaggregation of Revenue [Line Items]
Utility	610
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	633,720	652,833
Wholesale Energy Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	633,720	652,833
Wholesale Energy Operations | Operating Segments | Gas
Disaggregation of Revenue [Line Items]
Utility	633,720	652,833
Retail Gas and Other Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	0	75,651
Retail Gas and Other Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	0	75,651
Retail Gas and Other Operations | Operating Segments | Gas
Disaggregation of Revenue [Line Items]
Utility	0	75,651
Retail Electric Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	56,899	124,245
Retail Electric Operations | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Utility	14,164	29,762
Retail Electric Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	42,735	94,483
Retail Electric Operations | Operating Segments | Electric
Disaggregation of Revenue [Line Items]
Utility	56,899	124,245
On-Site Energy Production | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	48,748	72,374
On-Site Energy Production | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	48,748	72,374
On-Site Energy Production | Operating Segments | Solar
Disaggregation of Revenue [Line Items]
Utility	15,111	35,444
On-Site Energy Production | Operating Segments | CHP
Disaggregation of Revenue [Line Items]
Utility	27,993	30,473
On-Site Energy Production | Operating Segments | Landfills
Disaggregation of Revenue [Line Items]
Utility	5,644	6,457
Appliance Service Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,042	1,957
Appliance Service Operations | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Utility	2,042	1,957
Appliance Service Operations | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Utility	2,042	1,957
SJG Utility Operations
Disaggregation of Revenue [Line Items]
Utility	485,000	475,500
SJG Utility Operations | Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	485,012	475,497
SJG Utility Operations | Utility Operations | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Utility	356,646	329,207
SJG Utility Operations | Utility Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	116,959	132,055
SJG Utility Operations | Utility Operations | Operating Segments | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Utility	8,951	11,536
SJG Utility Operations | Utility Operations | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Utility	2,456	2,699
SJG Utility Operations | Utility Operations | Operating Segments | Gas
Disaggregation of Revenue [Line Items]
Utility	485,012	475,497
ETG Utility Operations | Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	331,027	128,647
ETG Utility Operations | Utility Operations | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Utility	217,195	82,763
ETG Utility Operations | Utility Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	103,590	42,935
ETG Utility Operations | Utility Operations | Operating Segments | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Utility	0	0
ETG Utility Operations | Utility Operations | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Utility	10,242	2,949
ETG Utility Operations | Utility Operations | Operating Segments | Gas
Disaggregation of Revenue [Line Items]
Utility	331,027	128,647
ELK Utility Operations | Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	7,857	3,362
ELK Utility Operations | Utility Operations | Operating Segments | Residential
Disaggregation of Revenue [Line Items]
Utility	3,494	1,482
ELK Utility Operations | Utility Operations | Operating Segments | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Utility	4,197	1,815
ELK Utility Operations | Utility Operations | Operating Segments | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Utility	0	0
ELK Utility Operations | Utility Operations | Operating Segments | Other
Disaggregation of Revenue [Line Items]
Utility	166	65
ELK Utility Operations | Utility Operations | Operating Segments | Gas
Disaggregation of Revenue [Line Items]
Utility	$ 7,857	$ 3,362