Note 8 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Estimate of Fair Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	$ 10.4	$ 6.7
Reported Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	11.2	6.7
Olkaria III DEG [Member]
Fair value levels-secured, unsecured and long term debt	16.3	24.2
Olkaria III DEG [Member] | Estimate of Fair Value Measurement [Member]
Loans	16.3	24.2
Olkaria III DEG [Member] | Reported Value Measurement [Member]
Loans	15.8	23.7
Olkaria III OPIC [Member]
Fair value levels-secured, unsecured and long term debt	253.4	262.6
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	253.4	262.6
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	246.6	264.6
Olkaria IV Loan - DEG 2 [Member]
Fair value levels-secured, unsecured and long term debt	50.9
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	50.9
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	50.0
Amatitlan Loan [Member]
Fair value levels-secured, unsecured and long term debt	37.3	41.7
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	37.3	41.7
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	36.8	40.3
Ormat Funding Corp [Member]
Fair value levels-secured, unsecured and long term debt	17.0	30.0
Ormat Funding Corp [Member] | Estimate of Fair Value Measurement [Member]
Notes	17.0	30.0
Ormat Funding Corp [Member] | Reported Value Measurement [Member]
Notes	17.0	30.0
Orcal Geothermal Inc [Member]
Fair value levels-secured, unsecured and long term debt	37.4	43.8
Orcal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	37.4	43.8
Orcal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	35.2	43.3
OFC Two Senior Secured Notes [Member]
Fair value levels-secured, unsecured and long term debt	249.0	231.1
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	249.0	231.1
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	247.2	262.0
Don A. Campbell 1 ("DAC1") [Member]
Fair value levels-secured, unsecured and long term debt	88.9
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	88.9
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	92.4
Senior Unsecured Bonds [Member]
Fair value levels-secured, unsecured and long term debt	200.1	264.5
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	200.1	264.5
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 204.3	$ 250.0