Note 7 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 5.4	$ 7.0
Reported Value Measurement [Member]
Other long-term debt	6.2	7.9
Olkaria III OPIC [Member]
Loans	211.8	234.6
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	211.8	234.6
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	210.6	228.6
Olkaria IV Loan - DEG 2 [Member]
Loans	47.2
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	47.2	50.7
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	47.5	50.0
Platanares Loan - OPIC [Member]
Loans	119.1
Platanares Loan - OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	119.1
Platanares Loan - OPIC [Member] | Reported Value Measurement [Member]
Loans	112.7
Amatitlan Loan [Member]
Loans	29.9	32.8
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	29.9	32.8
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	29.8	33.3
OrCal Geothermal Inc [Member]
Notes	19.0	34.2
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	19.0	34.2
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	18.7	32.1
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	214.5	234.6
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	217.8	232.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	78.8	85.5
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	78.8	85.5
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	83.3	88.3
USG Prudential - NV [Member]
Notes	29.4
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	29.4
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	27.8
USG Prudential - ID [Member]
Notes	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.9
USG DOE [Member]
Notes	48.3
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	48.3
USG DOE [Member] | Reported Value Measurement [Member]
Notes	51.4
Senior Unsecured Bonds [Member]
Senior Unsecured Bonds	199.4	200.3
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	199.4	200.3
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	204.3	204.3
Senior Unsecured Loan [Member]
Senior Unsecured Bonds	102.2
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured Bonds	102.2
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior Unsecured Bonds	$ 100.0