AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	$ 1,109	$ 1,420
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	7,835	6,024
SJRG
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	1,029	1,284
SJRG*	126	3,427
Marina
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	60	116
SJI
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	5,610	4,726
SJIU
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	955	0
Millennium
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	827	763
Other
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	20	20
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	$ 443	$ 535