Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 7.3	$ 10.4
Reported Value Measurement [Member]
Other long-term debt	8.0	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	242.9	253.4
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	242.9	253.4
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	233.1	246.6
Olkaria IV Loan - DEG 2 [Member]
Loans	52.4	50.9
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	52.4	50.9
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	50.0	50.0
Amatitlan Loan [Member]
Loans	34.2	37.3
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	34.2	37.3
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	34.1	36.8
Ormat Funding Corp [Member]
Notes		17.0
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	34.1	37.4
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	34.1	37.4
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	32.1	35.2
OFC Two Senior Secured Notes [Member]
Notes	242.5	249.0
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	242.5	249.0
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	236.6	247.2
Don A. Campbell 1 ("DAC1") [Member]
Notes	88.2	88.9
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	88.2	88.9
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	89.6	92.4
Senior Unsecured Bonds [Member]
Notes		200.1
Senior Unsecured Bonds	201.4
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	201.4	200.1
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 204.3	$ 204.3