Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 5.3	$ 5.4
Reported Value Measurement [Member]
Other long-term debt	6.2	6.2
Olkaria III OPIC [Member]
Loans	210.0	211.8
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	210.0	211.8
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	206.1	210.6
Olkaria IV Loan - DEG 2 [Member]
Loans	48.6
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	48.6	47.2
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	47.5	47.5
Olkaria IV Loan - DEG 3 [Member]
Loans	42.5
Olkaria IV Loan - DEG 3 [Member] | Estimate of Fair Value Measurement [Member]
Loans	42.5
Olkaria IV Loan - DEG 3 [Member] | Reported Value Measurement [Member]
Loans	41.5
Platanares Loan - OPIC [Member]
Loans	118.5	119.1
Platanares Loan - OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	118.5	119.1
Platanares Loan - OPIC [Member] | Reported Value Measurement [Member]
Loans	110.6	112.7
Amatitlan Loan [Member]
Loans	29.0	29.9
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	29.0	29.9
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	28.9	29.8
OrCal Geothermal Inc [Member]
Notes	19.2	19.0
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	19.2	19.0
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	18.7	18.7
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	213.4	214.5
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	213.2	217.8
Don A. Campbell 1 ("DAC1") [Member]
Notes	78.6	78.8
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	78.6	78.8
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	81.7	83.3
USG Prudential - NV [Member]
Notes	29.9	29.4
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	29.9	29.4
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	27.8	27.8
USG Prudential - ID [Member]
Notes	18.0	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.0	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.4	18.9
USG DOE [Member]
Notes	47.3	48.3
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	47.3	48.3
USG DOE [Member] | Reported Value Measurement [Member]
Notes	49.8	51.4
Senior Unsecured Bonds [Member]
Senior Unsecured debt	199.3	199.4
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	199.3	199.4
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior Unsecured debt	153.6	102.2
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	153.6	102.2
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	$ 150.0	$ 100.0