LEASES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jan. 01, 2019
Lessee, Lease, Description [Line Items]
Percentage of operating leases which are real estate leases		64.00%
Operating lease right-of-use asset			$ 3,100
Operating Lease, Liability	$ 1,871	$ 1,871	3,100
Operating lease cost	700	2,300
Operating lease payments due	400	1,200
Variable lease cost	$ 300	$ 1,100
Weighted-average remaining lease term - operating leases	2 years 7 months 6 days	2 years 7 months 6 days
Weighted-average discount rate - operating leases	3.00%	3.00%
Thermal Energy Generating Property and Equipment | Marina
Lessee, Lease, Description [Line Items]
Operating lease right-of-use asset	$ 69,100	$ 69,100
Operating lease cost		40,100
Real Estate
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	1,300	1,300
Equipment
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	700	700
Vehicles
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	$ 100	$ 100
Minimum
Lessee, Lease, Description [Line Items]
Renewal term of operating lease	1 year	1 year
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
Lessee, Lease, Description [Line Items]
Renewal term of operating lease	5 years	5 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		38.00%
Operating lease right-of-use asset	$ 400	$ 400	500
Operating Lease, Liability	354	354	$ 500
Operating lease cost	100	300
Operating lease payments due	$ 100	$ 300
Weighted-average remaining lease term - operating leases	6 years 10 months 24 days	6 years 10 months 24 days
Weighted-average discount rate - operating leases	3.00%	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	1,900	1,900
Operating Lease, Liability	$ 2,100	$ 2,100