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Note 7 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Estimate of Fair Value Measurement [Member]    
Other long-term debt $ 7.0 $ 10.4
Reported Value Measurement [Member]    
Other long-term debt 7.9 11.2
Olkaria III DEG [Member]    
Loans   16.3
Olkaria III DEG [Member] | Estimate of Fair Value Measurement [Member]    
Loans 16.3
Olkaria III DEG [Member] | Reported Value Measurement [Member]    
Loans 15.8
Olkaria III OPIC [Member]    
Loans 234.6 253.4
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]    
Loans 234.6 253.4
Olkaria III OPIC [Member] | Reported Value Measurement [Member]    
Loans 228.6 246.6
Olkaria IV Loan - DEG 2 [Member]    
Loans 50.7 50.9
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]    
Loans 50.7 50.9
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]    
Loans 50.0 50.0
Amatitlan Loan [Member]    
Loans 32.8 37.3
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]    
Loans 32.8 37.3
Amatitlan Loan [Member] | Reported Value Measurement [Member]    
Loans 33.3 36.8
Ormat Funding Corp [Member] | Estimate of Fair Value Measurement [Member]    
Notes 17.0
Ormat Funding Corp [Member] | Reported Value Measurement [Member]    
Notes 17.0
OrCal Geothermal Inc [Member]    
Notes 34.2 37.4
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]    
Notes 34.2 37.4
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]    
Notes 32.1 35.2
OFC Two Senior Secured Notes [Member]    
Notes 234.6 249.0
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]    
Notes 234.6 249.0
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]    
Notes 232.5 247.2
Don A. Campbell 1 ("DAC1") [Member]    
Notes 85.5 88.9
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]    
Notes 85.5 88.9
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]    
Notes 88.3 92.4
Senior Unsecured Bonds [Member]    
Senior Unsecured Bonds 200.3 200.1
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]    
Senior Unsecured Bonds 200.3 200.1
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]    
Senior Unsecured Bonds $ 204.3 $ 204.3