Note 12 - Puna Power Plant Lease Transactions (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2005	Dec. 31, 2018	Dec. 31, 2016
Lease Term		31 years
Deferred Revenue, Leases, Gross		$ 83,000,000
Capital Leases, Balance Sheet, Assets by Major Class, Net, Total	$ 25,300,000		$ 19,700,000
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation	$ 35,600,000		33,100,000
Lease Payment Term	23 years
Deferred Costs, Leasing, Gross	$ 4,200,000
Restricted Cash and Cash Equivalents, Total	48,825,000		78,693,000	$ 34,262,000
Cash, Ending Balance	0		0
Lease Agreement, Reserve Funds [Member]
Restricted Cash and Cash Equivalents, Total	$ 7,900,000		$ 1,300,000