Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Estimate of Fair Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	$ 8.9	$ 10.4
Reported Value Measurement [Member]
Fair value levels-secured, unsecured and long term debt	9.7	11.2
Olkaria III DEG [Member]
Fair value levels-secured, unsecured and long term debt	12.5	16.3
Olkaria III DEG [Member] | Estimate of Fair Value Measurement [Member]
Loans	12.5	16.3
Olkaria III DEG [Member] | Reported Value Measurement [Member]
Loans	11.8	15.8
Olkaria III OPIC [Member]
Fair value levels-secured, unsecured and long term debt	247.8	253.4
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	247.8	253.4
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	237.6	246.6
Olkaria IV Loan - DEG 2 [Member]
Fair value levels-secured, unsecured and long term debt	51.7	50.9
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	51.7	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	35.1	37.3
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	35.1	37.3
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	35.0	36.8
Ormat Funding Corp [Member]
Fair value levels-secured, unsecured and long term debt	14.3	17.0
Ormat Funding Corp [Member] | Estimate of Fair Value Measurement [Member]
Notes	14.3	17.0
Ormat Funding Corp [Member] | Reported Value Measurement [Member]
Notes	14.3	17.0
Orcal Geothermal Inc [Member]
Fair value levels-secured, unsecured and long term debt	33.8	37.4
Orcal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	33.8	37.4
Orcal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	32.1	35.2
OFC Two Senior Secured Notes [Member]
Fair value levels-secured, unsecured and long term debt	245.1	249.0
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	245.1	249.0
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	238.9	247.2
Don A. Campbell 1 ("DAC1") [Member]
Fair value levels-secured, unsecured and long term debt	88.7	88.9
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	88.7	88.9
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	90.2	92.4
Senior Unsecured Bonds [Member]
Fair value levels-secured, unsecured and long term debt	203.3	200.1
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	203.3	200.1
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 204.3	$ 204.3