AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	$ 615	$ 1,194	$ 2,311	$ 3,868
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	7,919	6,851	23,875	26,386
SJRG
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	595	1,092	2,192	3,518
SJRG*	1,808	2,326	3,404	8,908
Marina
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	0	82	60	290
SJI
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	5,909	5,087	17,463	15,507
SJIU
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	743	630	2,616	1,971
Millennium
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	814	794	2,461	2,118
Other
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	20	20	59	60
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	$ 453	$ 340	$ 1,335	$ 6,790