AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details) - SJG - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	$ 588	$ 1,267	$ 1,697	$ 2,687
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	8,121	7,487	15,956	13,511
SJRG
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	568	1,142	1,597	2,426
SJRG*	1,470	3,335	1,596	6,582
Marina
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	0	105	60	221
SJI
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	5,944	5,694	11,554	10,420
SJIU
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	917	578	1,873	578
Millennium
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	821	789	1,647	1,552
Other
Regulated Operating Revenue [Abstract]
Total Operating Revenue/Affiliates	20	20	40	40
Operating Expenses [Abstract]
Total Operations Expense/Affiliates	$ 439	$ 426	$ 882	$ 961