AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	$ 928	$ 615	$ 7,724	$ 2,311
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	7,752	7,919	22,800	23,875
SJRG
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	907	595	7,662	2,192
Cost of sales/affiliates	954	1,808	4,924	3,404
Marina
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	0	0	0	60
SJI
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	5,832	5,909	17,082	17,463
SJIU
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	1,045	743	2,957	2,616
Millennium
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	857	814	2,598	2,461
Other
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	21	20	62	59
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	$ 18	$ 453	$ 163	$ 1,335