REVENUE - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Utility	$ 315,148	$ 268,843	$ 1,077,434	$ 1,107,360
Gas
Disaggregation of Revenue [Line Items]
Utility	303,198	243,788	1,043,638	1,026,780
Electric
Disaggregation of Revenue [Line Items]
Utility	4,490	12,296	15,476	40,688
Solar
Disaggregation of Revenue [Line Items]
Utility	5,429	3,429	8,725	12,443
CHP
Disaggregation of Revenue [Line Items]
Utility		7,218	3,502	21,371
Landfills
Disaggregation of Revenue [Line Items]
Utility	888	1,333	3,232	4,284
Other Product
Disaggregation of Revenue [Line Items]
Utility	1,143	779	2,861	1,794
Residential
Disaggregation of Revenue [Line Items]
Utility	62,442	52,310	401,274	403,172
Commercial and Industrial
Disaggregation of Revenue [Line Items]
Utility	250,252	211,999	664,471	690,120
OSS and Capacity Release
Disaggregation of Revenue [Line Items]
Utility	1,859	2,234	5,931	6,248
Other Customer
Disaggregation of Revenue [Line Items]
Utility	595	2,300	5,758	7,820
SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	67,765	55,198	352,127	335,179
SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	67,765	55,198	352,127	335,179
SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	40,900	31,256	244,114	244,889
SJG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	24,625	21,164	100,689	82,255
SJG Utility Operations | OSS and Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,859	2,234	5,931	6,248
SJG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	381	544	1,393	1,787
ETG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	38,163	30,624	232,410	220,503
ETG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	38,163	30,624	232,410	220,503
ETG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	20,974	16,092	153,496	143,584
ETG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	16,998	12,799	74,752	71,013
ETG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	191	1,733	4,162	5,906
ELK Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	285	895	4,926	5,100
ELK Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	285	895	4,926	5,100
ELK Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	77	243	2,179	2,204
ELK Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	185	629	2,544	2,769
ELK Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	23	23	203	127
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	197,580	158,244	456,427	469,805
Wholesale Energy Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	197,580	158,244	456,427	469,805
Wholesale Energy Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	197,580	158,244	456,427	469,805
Retail Electric Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	4,814	14,480	17,462	46,663
Retail Electric Operations | Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	4,814	14,480	17,462	46,663
Retail Electric Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility		4,205		10,966
Retail Electric Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	4,814	10,275	17,462	35,697
On-Site Energy Production | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	6,317	11,980	15,459	38,098
On-Site Energy Production | Solar | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	5,429	3,429	8,725	12,443
On-Site Energy Production | CHP | Operating Segments
Disaggregation of Revenue [Line Items]
Utility		7,218	3,502	21,371
On-Site Energy Production | Landfills | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	888	1,333	3,232	4,284
On-Site Energy Production | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	6,317	11,980	15,459	38,098
Appliance Service Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	491	514	1,485	1,529
Appliance Service Operations | Other Product | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	491	514	1,485	1,529
Appliance Service Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	491	514	1,485	1,529
Corporate and Services | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Utility	(267)	(3,092)	(2,862)	(9,517)
Corporate and Services | Gas | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Utility	(595)	(1,173)	(2,252)	(3,807)
Corporate and Services | Electric | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Utility	(324)	(2,184)	(1,986)	(5,975)
Corporate and Services | Other Product | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Utility	652	265	1,376	265
Corporate and Services | Commercial and Industrial | Corporate Services And Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Utility	$ (267)	$ (3,092)	$ (2,862)	$ (9,517)