REVENUE - Disaggregated Revenue (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,510,174
Utility	1,510,174
Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	445,171
Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,047,754
OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11,536
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5,713
Gas
Disaggregation of Revenue [Line Items]
Utility	1,325,809
Electric
Disaggregation of Revenue [Line Items]
Utility	116,341
Solar
Disaggregation of Revenue [Line Items]
Utility	29,137
CHP
Disaggregation of Revenue [Line Items]
Utility	30,473
Landfills
Disaggregation of Revenue [Line Items]
Utility	6,457
Other
Disaggregation of Revenue [Line Items]
Utility	1,957
Wholesale Energy Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	652,833
Utility	652,833
Wholesale Energy Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	652,833
Wholesale Energy Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	652,833
Retail Gas and Other Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	75,651
Utility	75,651
Retail Gas and Other Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	75,651
Retail Gas and Other Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	75,651
Retail Electric Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	124,245
Utility	124,245
Retail Electric Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	29,762
Retail Electric Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	94,483
Retail Electric Operations | Electric
Disaggregation of Revenue [Line Items]
Utility	124,245
On-Site Energy Production
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	72,374
Utility	72,374
On-Site Energy Production | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	72,374
On-Site Energy Production | Solar
Disaggregation of Revenue [Line Items]
Utility	35,444
On-Site Energy Production | CHP
Disaggregation of Revenue [Line Items]
Utility	30,473
On-Site Energy Production | Landfills
Disaggregation of Revenue [Line Items]
Utility	6,457
Appliance Service Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,957
Utility	1,957
Appliance Service Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,957
Appliance Service Operations | Other
Disaggregation of Revenue [Line Items]
Utility	1,957
Corporate Services and Other Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	(24,392)
Utility	(24,392)
Corporate Services and Other Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	(24,392)
Corporate Services and Other Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	(10,181)
Corporate Services and Other Operations | Electric
Disaggregation of Revenue [Line Items]
Utility	(7,904)
Corporate Services and Other Operations | Solar
Disaggregation of Revenue [Line Items]
Utility	(6,307)
SJG Utility Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	475,500
SJG Utility Operations | Utility Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	475,497
Utility	475,497
SJG Utility Operations | Utility Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	329,207
SJG Utility Operations | Utility Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	132,055
SJG Utility Operations | Utility Operations | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11,536
SJG Utility Operations | Utility Operations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,699
SJG Utility Operations | Utility Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	475,497
ETG Utility Operations | Utility Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	128,647
Utility	128,647
ETG Utility Operations | Utility Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	82,763
ETG Utility Operations | Utility Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	42,935
ETG Utility Operations | Utility Operations | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax
ETG Utility Operations | Utility Operations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2,949
ETG Utility Operations | Utility Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	128,647
ELK Utility Operations | Other
Disaggregation of Revenue [Line Items]
Utility
ELK Utility Operations | Utility Operations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3,362
Utility	3,362
ELK Utility Operations | Utility Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,482
ELK Utility Operations | Utility Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,815
ELK Utility Operations | Utility Operations | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0
ELK Utility Operations | Utility Operations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	65
ELK Utility Operations | Utility Operations | Gas
Disaggregation of Revenue [Line Items]
Utility	$ 3,362