AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	$ 1,194	$ 803	$ 3,868	$ 4,738
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	6,851	6,786	26,386	21,746
SJRG
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	1,092	691	3,518	4,388
SJRG*	2,326	1,094	8,908	28,525
Marina
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	82	89	290	281
SJI
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	5,087	6,148	15,507	19,899
SJIU
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	630	0	1,971	0
Millennium
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	794	750	2,118	2,191
Other
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	20	23	60	69
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	$ 340	$ (112)	$ 6,790	$ (344)