AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	$ 6,350	$ 1,109
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	7,463	7,835
SJRG
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	6,329	1,029
SJRG	1,367	126
Marina
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	0	60
SJI
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	5,564	5,610
SJIU
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	960	955
Millennium
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	866	827
Other
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	21	20
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	$ 73	$ 443