Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Sep. 30, 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	6.4	7.9
Olkaria III OPIC [Member]
Loans	214.2	234.6
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	214.2	234.6
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	215.1	228.6
Olkaria IV Loan - DEG 2 [Member]
Loans	50.1
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	50.1	50.7
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	50.0	50.0
Amatitlan Loan [Member]
Loans	30.8	32.8
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	30.8	32.8
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	30.6	33.3
OrCal Geothermal Inc [Member]
Notes	24.7	34.2
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	24.7	34.2
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	24.0	32.1
OFC Two Senior Secured Notes [Member]
Notes	215.9	234.6
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	215.9	234.6
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	221.8	232.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	79.0	85.5
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	79.0	85.5
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	84.7	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	28.2
USG Prudential - ID [Member]
Notes	18.7
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.7
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.9
USG DOE [Member]
Notes	47.2
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	47.2
USG DOE [Member] | Reported Value Measurement [Member]
Notes	51.4
Senior Unsecured Bonds [Member]
Senior secured debt	195.7	200.3
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior secured debt	195.7	200.3
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior secured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior secured debt	99.7
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior secured debt	99.7
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior secured debt	$ 100.0