AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	$ 447	$ 588	$ 6,796	$ 1,697
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	7,584	8,121	15,046	15,956
SJRG
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	426	568	6,755	1,597
SJRG	2,603	1,470	3,970	1,596
Marina
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	0	0	0	60
SJI
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	5,685	5,944	11,249	11,554
SJIU
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	952	917	1,912	1,873
Millennium
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	874	821	1,740	1,647
Other
Operating Revenues/Affiliates:
Total Operating Revenue/Affiliates	21	20	41	40
Operations Expense/Affiliates:
Total Operations Expense/Affiliates	$ 73	$ 439	$ 145	$ 882