REVENUE - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Utility	$ 274,714	$ 231,840	$ 750,669	$ 838,517
Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(592)	(3,418)	(2,596)	(6,425)
Gas
Disaggregation of Revenue [Line Items]
Utility	266,425	203,380	728,824	782,992
Gas | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(568)	(1,234)	(1,657)	(2,634)
Electric
Disaggregation of Revenue [Line Items]
Utility	5,275	13,188	10,986	28,392
Electric | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(379)	(2,184)	(1,662)	(3,791)
Solar
Disaggregation of Revenue [Line Items]
Utility	1,390	6,438	3,296	9,014
Solar | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	0
CHP
Disaggregation of Revenue [Line Items]
Utility		6,794	3,502	14,153
Landfills
Disaggregation of Revenue [Line Items]
Utility	764	1,556	2,344	2,951
Other Product
Disaggregation of Revenue [Line Items]
Utility	860	484	1,717	1,015
Other Product | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	355		723
Residential
Disaggregation of Revenue [Line Items]
Utility	100,148	59,826	337,390	350,862
Commercial and Industrial
Disaggregation of Revenue [Line Items]
Utility	172,431	167,633	404,046	478,120
Commercial and Industrial | Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Utility	(592)	(3,418)	(2,596)	(6,425)
OSS and Capacity Release
Disaggregation of Revenue [Line Items]
Utility	1,463	2,244	4,072	4,014
Other Customer
Disaggregation of Revenue [Line Items]
Utility	672	2,137	5,161	5,521
Wholesale Energy Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	119,852	111,794	258,847	311,561
Wholesale Energy Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	119,852	111,794	258,847	311,561
Wholesale Energy Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	119,852	111,794	258,847	311,561
Retail Electric Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	5,654	15,372	12,648	32,183
Retail Electric Operations | Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	5,654	15,372	12,648	32,183
Retail Electric Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility		2,953	0	6,761
Retail Electric Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	5,654	12,419	12,648	25,422
On-Site Energy Production | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,154	14,788	9,142	26,118
On-Site Energy Production | Solar | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,390	6,438	3,296	9,014
On-Site Energy Production | CHP | Operating Segments
Disaggregation of Revenue [Line Items]
Utility		6,794	3,502	14,153
On-Site Energy Production | Landfills | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	764	1,556	2,344	2,951
On-Site Energy Production | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	2,154	14,788	9,142	26,118
Appliance Service Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	505	484	994	1,015
Appliance Service Operations | Other Product | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	505	484	994	1,015
Appliance Service Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	505	484	994	1,015
Appliance Service Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			0
SJG | SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	84,937	46,997	272,746	279,981
SJG | SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	84,937	46,997	272,746	279,981
SJG | SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	58,964	28,679	201,772	213,633
SJG | SJG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	24,057	15,503	65,892	61,091
SJG | SJG Utility Operations | OSS and Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,463	2,244	4,072	4,014
SJG | SJG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	453	571	1,010	1,243
ETG Utility Operations | SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	60,841	45,095	194,247	189,879
ETG Utility Operations | SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	60,841	45,095	194,247	189,879
ETG Utility Operations | SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	40,113	27,503	132,522	127,492
ETG Utility Operations | SJG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	20,606	16,065	57,754	58,213
ETG Utility Operations | SJG Utility Operations | OSS and Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			0
ETG Utility Operations | SJG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	122	1,527	3,971	4,174
ELK Utility Operations | SJG Utility Operations | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,363	728	4,641	4,205
ELK Utility Operations | SJG Utility Operations | Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	1,363	728	4,641	4,205
ELK Utility Operations | SJG Utility Operations | Residential | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	566	207	2,102	1,961
ELK Utility Operations | SJG Utility Operations | Commercial and Industrial | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	700	482	2,359	2,140
ELK Utility Operations | SJG Utility Operations | OSS and Capacity Release | Operating Segments
Disaggregation of Revenue [Line Items]
Utility			0
ELK Utility Operations | SJG Utility Operations | Other Customer | Operating Segments
Disaggregation of Revenue [Line Items]
Utility	$ 97	$ 39	$ 180	$ 104