Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Olkaria III DEG [Member] | Estimate of Fair Value Measurement [Member]
Loans	$ 20.8	$ 24.2
Olkaria III DEG [Member] | Reported Value Measurement [Member]
Loans	19.7	23.7
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	258.3	262.6
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	255.6	264.6
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	39.7	41.7
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	38.5	40.3
Ormat Funding Corp [Member] | Estimate of Fair Value Measurement [Member]
Notes	26.5	30.0
Ormat Funding Corp [Member] | Reported Value Measurement [Member]
Notes	26.5	30.0
Orcal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	41.4	43.8
Orcal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	40.1	43.3
Ofc Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	229.0	231.1
Ofc Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	253.6	262.0
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	259.0	264.5
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 249.8	$ 250.0