LEASES - OPERATING (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Operating Leases, Future Minimum Payments Receivable [Abstract]
2019	$ 5,396
2020	5,396
2021	5,396
2022	5,396
2023	5,396
Thereafter	18,438
Total minimum future rentals	45,418
Thermal energy generating property and equipment
Property Subject to or Available for Operating Lease, Net [Abstract]
Property and equipment under operating lease	71,500	$ 74,200
Accumulated depreciation of property and equipment under operating lease	$ 37,700	$ 34,500