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Note 1 - General and Basis of Presentation (Details Textual)
3 Months Ended 6 Months Ended 12 Months Ended
May 17, 2018
USD ($)
Apr. 24, 2018
USD ($)
MWh
Mar. 22, 2018
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
May 03, 2018
USD ($)
Insured Event, Gain (Loss)           $ 7,150,000    
Property, Plant and Equipment, Net, Ending Balance       $ 1,840,558,000   1,840,558,000   $ 1,734,691,000  
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax       5,000,000 $ 30,000,000 10,000,000 54,000,000    
Interest Expense, Total       15,846,000 14,540,000 30,190,000 29,463,000    
Income Tax Expense (Benefit), Total       29,105,000 32,765,000 2,163,000 43,769,000    
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total       (2,297,000)   (2,297,000)   (4,706,000)  
Exploration Abandonment and Impairment Expense       0 $ 0 119,000 $ 0    
Accounts Receivable, Net, Current, Total       $ 109,061,000   109,061,000   $ 110,410,000  
Provision for Doubtful Accounts           $ 0      
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]                  
Concentration Risk, Percentage           55.00%   57.00%  
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]                  
Concentration Risk, Percentage       17.00% 16.70% 16.70% 17.80%    
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]                  
Concentration Risk, Percentage       14.90% 8.70% 15.60% 8.90%    
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power and Lighting Co LTD [Member]                  
Concentration Risk, Percentage       16.60% 15.40% 15.80% 14.80%    
UNITED STATES                  
Cash, Cash Equivalents, and Short-term Investments, Total       $ 21,600,000   $ 21,600,000   $ 21,200,000  
Foreign Countries [Member]                  
Cash, Cash Equivalents, and Short-term Investments, Total       59,800,000   59,800,000   32,800,000  
Accounts Receivable, Net, Current, Total       83,200,000   83,200,000   $ 78,100,000  
Other Comprehensive Income (Loss) [Member]                  
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total       1,000,000   1,000,000      
Reclassification out of Accumulated Other Comprehensive Income [Member]                  
Interest Expense, Total       6,000,000 $ 20,000,000 (15,000,000) $ (30,000,000)    
Income Tax Expense (Benefit), Total       (1,000,000) $ 10,000,000 5,000,000 $ 24,000,000    
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       3,400,000   3,400,000      
Business Acquisition, Pro Forma Operating Income (Loss) since Acquisition Date, Actual       (4,200,000)   $ (4,200,000)      
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   9              
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          
Property, Plant and Equipment, Net, Ending Balance                 $ 102,200,000
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%