Note 1 - General and Basis of Presentation (Details Textual)				3 Months Ended			9 Months Ended		12 Months Ended
	May 17, 2018 USD ($)	Apr. 24, 2018 USD ($) MWh	Mar. 22, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Insured Event, Gain (Loss)							$ 7,150,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				$ 6,000,000		$ 2,000,000	16,000,000	56,000,000
Interest Expense, Total				18,700,000		11,692,000	48,890,000	41,155,000
Income Tax Expense (Benefit), Total				1,184,000		6,224,000	3,347,000	49,993,000
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total				(1,386,000)			(1,386,000)		$ (4,706,000)
Exploration Abandonment and Impairment Expense				0		$ 0	119,000	$ 0
Accounts Receivable, Net, Current, Total				$ 118,675,000			118,675,000		$ 110,410,000
Provision for Doubtful Accounts							$ 0
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage							53.00%		57.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage				13.60%		16.30%	15.70%	17.40%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]
Concentration Risk, Percentage				13.70%		9.10%	14.90%	8.90%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power and Lighting Co LTD [Member]
Concentration Risk, Percentage				18.60%		17.60%	16.70%	15.70%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments, Total				$ 20,900,000			$ 20,900,000		$ 21,200,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments, Total				76,800,000			76,800,000		32,800,000
Accounts Receivable, Net, Current, Total				92,600,000			92,600,000		$ 78,100,000
Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total				900,000			900,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest Expense, Total				6,000,000		$ (9,000,000)	22,000,000	$ 21,000,000
Income Tax Expense (Benefit), Total				0		$ (11,000,000)	5,000,000	$ (35,000,000)
Migdal Loan [Member]
Debt Instrument, Face Amount			$ 100,000,000
Debt Instrument, Periodic Payment, Total			4,200,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid			$ 37,000,000
Debt Instrument, Interest Rate, Stated Percentage			4.80%
Debt Instrument Increase in Stated Interest Rate if Rating is Downgraded to ILA Negative			0.50%
Debt Instrument Increase in Stated Interest Rate Each Additional Downgrade			0.25%
Debt Instrument Decrease in Stated Interest Rate for Each Rating Upgrade			0.25%
Debt to EBITDA Ratio Threshold for Rate Increase			4.5
Debt Instrument Increase in Stated Interest Rate if Debt to EBITDA Ratio Exceeds Threshold			0.50%
Debt to EBITDA Ratio Requirement			6
Stockholders Equity, Debt Covenant, Minimum Threshold			$ 650,000,000
Stockholders Equity to Total Assets, Ratio			25.00%
Stockholders Equity, Debt Covenant, Minimum Threshold to Pay Dividends			$ 800,000,000
Dividends to Net Income, Ratio			50.00%
Migdal Loan [Member] | Maximum [Member]
Debt Instrument, Increase in Stated Interest Rate			1.00%
Migdal Loan [Member] | Minimum [Member]
Debt Instrument, Interest Rate, Stated Percentage			4.80%
U.S. Geothermal [Member]
Payments to Acquire Businesses, Gross		$ 110,000,000
Current Power Generation | MWh		38
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual				7,300,000			10,700,000
Business Acquisition, Pro Forma Operating Income (Loss) since Acquisition Date, Actual				1,600,000			(2,600,000)
U.S. Geothermal [Member] | Neal Hot Springs [Member]
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value		$ 34,900,000
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Percent		40.00%
U.S. Geothermal [Member] | Neal Hot Springs [Member] | Measurement Input, Expected Term [Member]
Noncontrolling Interest, Measurement Input		20
U.S. Geothermal [Member] | Neal Hot Springs [Member] | Measurement Input, Discount Rate [Member]
Noncontrolling Interest, Measurement Input		0.09
U.S. Geothermal [Member] | Long-term Electricity Power Purchase Agreements [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		19 years
U.S. Geothermal [Member] | Ormat Nevada Inc. [Member]
Payments to Acquire Businesses, Gross		$ 106,000,000
U.S. Geothermal [Member] | Ormat Technologies, Inc. [Member]
Payments to Acquire Businesses, Gross		$ 4,000,000
Puna Geothermal Power Plant [Member]
Impairment of Long-Lived Assets Held-for-use					$ 4,900,000
Puna Geothermal Power Plant [Member] | Other Income [Member]
Insured Event, Gain (Loss)					$ 7,200,000
Tungsten Mountain [Member]
Parternship Agreement, Initial Purchase Price	$ 33,400,000
Partnership Agreement, Expected Additional Installments	$ 13,000,000
Partnership Agreement, Percentage of Distributable Cash Flow Generated	97.50%
Partnership Agreement, Percentage of Taxable Income	95.00%
Galena 2 Power Purchase Agreement [Member]
Termination Fees				$ 5,000,000			$ 5,000,000