LEASES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Percentage of operating leases which are real estate leases	86.00%
Operating Lease, Liability	$ 2,030
Operating lease cost	500	$ 800
Operating lease payments due	400	400
Variable lease cost	$ 100	400
Weighted-average remaining lease term - operating leases	2 years 10 months 24 days
Weighted-average discount rate - operating leases	3.00%
Real Estate
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	$ 1,700		$ 1,500
Equipment
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	200		300
Vehicles
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	$ 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
Lessee, Lease, Description [Line Items]
Renewal term of operating lease	1 year
Minimum | Real Estate
Lessee, Lease, Description [Line Items]
Term of operating lease	5 years
Minimum | Property, Plant and Equipment
Lessee, Lease, Description [Line Items]
Term of operating lease	3 years
Maximum
Lessee, Lease, Description [Line Items]
Renewal term of operating lease	5 years
Maximum | Real Estate
Lessee, Lease, Description [Line Items]
Term of operating lease	15 years
Maximum | Property, Plant and Equipment
Lessee, Lease, Description [Line Items]
Term of operating lease	25 years
SJG
Lessee, Lease, Description [Line Items]
Percentage of operating leases which are real estate leases	49.00%
Operating lease right-of-use asset	$ 300		300
Operating Lease, Liability	270		300
Operating lease cost	100	100
Operating lease payments due	$ 100	$ 100
Weighted-average remaining lease term - operating leases	7 years 8 months 12 days
Weighted-average discount rate - operating leases	3.00%
SJG | Real Estate
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	$ 100		100
SJG | Equipment
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	200		200
SJI
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset	$ 2,000		$ 1,900