Note 5 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Deposits	$ 15.9	$ 14.4
Revolving lines of credit	30.0
Fair Value, Inputs, Level 1 [Member]
Deposits	15.9	14.4
Revolving lines of credit
Fair Value, Inputs, Level 2 [Member]
Deposits
Revolving lines of credit	30.0
Fair Value, Inputs, Level 3 [Member]
Deposits
Revolving lines of credit
Olkaria III DEG [Member]
Fair value levels-secured, unsecured and long term debt	12.5	16.3
Olkaria III DEG [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria III DEG [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria III DEG [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	12.5	16.3
Olkaria III OPIC [Member]
Fair value levels-secured, unsecured and long term debt	247.8	253.4
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	247.8	253.4
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] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria IV Loan - DEG 2 [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Olkaria IV Loan - DEG 2 [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	51.7	50.9
Amatitlan Loan [Member]
Fair value levels-secured, unsecured and long term debt	35.1	37.3
Amatitlan Loan [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Amatitlan Loan [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt	35.1	37.3
Amatitlan Loan [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt
Ormat Funding Corp [Member]
Fair value levels-secured, unsecured and long term debt	14.3	17.0
Ormat Funding Corp [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Ormat Funding Corp [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt	14.3	17.0
Ormat Funding Corp [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt
Orcal Geothermal Inc [Member]
Fair value levels-secured, unsecured and long term debt	33.8	37.4
Orcal Geothermal Inc [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Orcal Geothermal Inc [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Orcal Geothermal Inc [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	33.8	37.4
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] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
OFC Two Senior Secured Notes [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
OFC Two Senior Secured Notes [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	245.1	249.0
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] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Don A. Campbell 1 ("DAC1") [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Don A. Campbell 1 ("DAC1") [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	88.7	88.9
Senior Unsecured Bonds [Member]
Fair value levels-secured, unsecured and long term debt	203.3	200.1
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	203.3	200.1
Other Long-term Debt [Member]
Fair value levels-secured, unsecured and long term debt	8.9	10.4
Other Long-term Debt [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value levels-secured, unsecured and long term debt
Other Long-term Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value levels-secured, unsecured and long term debt	1.7	3.3
Other Long-term Debt [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value levels-secured, unsecured and long term debt	$ 7.2	$ 7.1