Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Estimate of Fair Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	$ 8.6	$ 10.4
Reported Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	9.5	11.2
Olkaria III DEG [Member]
Fair value levels-secured, unsecured and long term debt	16.2	16.3
Olkaria III DEG [Member] | Estimate of Fair Value Measurement [Member]
Loans	16.2	16.3
Olkaria III DEG [Member] | Reported Value Measurement [Member]
Loans	15.8	15.8
Olkaria III OPIC [Member]
Fair value levels-secured, unsecured and long term debt	248.9	253.4
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	248.9	253.4
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	242.1	246.6
Olkaria IV Loan - DEG 2 [Member]
Fair value levels-secured, unsecured and long term debt	51.6	50.9
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	51.6	50.9
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	50.0	50.0
Amatitlan Loan [Member]
Fair value levels-secured, unsecured and long term debt	36.2	37.3
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	36.2	37.3
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	35.9	36.8
Ormat Funding Corp [Member]
Fair value levels-secured, unsecured and long term debt	17.0	17.0
Ormat Funding Corp [Member] | Estimate of Fair Value Measurement [Member]
Notes	17.0	17.0
Ormat Funding Corp [Member] | Reported Value Measurement [Member]
Notes	17.0	17.0
Orcal Geothermal Inc [Member]
Fair value levels-secured, unsecured and long term debt	37.7	37.4
Orcal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	37.7	37.4
Orcal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	35.2	35.2
OFC Two Senior Secured Notes [Member]
Fair value levels-secured, unsecured and long term debt	244.6	249.0
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	244.6	249.0
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	242.7	247.2
Don A. Campbell 1 ("DAC1") [Member]
Fair value levels-secured, unsecured and long term debt	87.4	88.9
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	87.4	88.9
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	90.8	92.4
Senior Unsecured Bonds [Member]
Fair value levels-secured, unsecured and long term debt	198.9	200.1
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	198.9	200.1
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 204.3	$ 204.3