LEASES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Percentage of operating leases which are real estate leases			89.00%
Operating lease liabilities	$ 1,879		$ 1,879
Operating lease cost	600	$ 800	1,100	$ 1,600
Operating lease payments due	400	400	700	800
Variable lease cost	$ 200	400	$ 300	800
Weighted-average remaining lease term - operating leases	4 years 4 months 24 days		4 years 4 months 24 days
Weighted-average discount rate - operating leases	3.10%		3.10%
Real Estate
Lessee, Lease, Description [Line Items]
Operating lease liabilities	$ 1,700		$ 1,700		$ 1,500
Equipment
Lessee, Lease, Description [Line Items]
Operating lease liabilities	200		200		300
Vehicles
Lessee, Lease, Description [Line Items]
Operating lease liabilities	$ 100		$ 100		100
Thermal Energy Generating Property and Equipment | Marina
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset					68,900
Operating lease cost					40,600
Minimum | Real Estate
Lessee, Lease, Description [Line Items]
Term of operating lease	5 years		5 years
Minimum | Property, Plant and Equipment
Lessee, Lease, Description [Line Items]
Term of operating lease	3 years		3 years
Maximum | Real Estate
Lessee, Lease, Description [Line Items]
Term of operating lease	15 years		15 years
Maximum | Property, Plant and Equipment
Lessee, Lease, Description [Line Items]
Term of operating lease	25 years		25 years
SJG
Lessee, Lease, Description [Line Items]
Percentage of operating leases which are real estate leases			56.00%
Operating lease right-of-use asset	$ 200		$ 200		300
Operating lease liabilities	229		229		300
Operating lease cost	100	100	100	200
Operating lease payments due	$ 100	$ 100	$ 100	$ 200
Weighted-average remaining lease term - operating leases	8 years 4 months 24 days		8 years 4 months 24 days
Weighted-average discount rate - operating leases	3.10%		3.10%
SJG | Real Estate
Lessee, Lease, Description [Line Items]
Operating lease liabilities	$ 100		$ 100		100
SJG | Equipment
Lessee, Lease, Description [Line Items]
Operating lease liabilities	100		100		200
SJI
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset	1,900		1,900		1,900
Operating lease liabilities	$ 1,900		$ 1,900		$ 1,900