Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Olkaria III OPIC [Member]
Loans	$ 184.2	$ 192.5
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	40.6
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	35.8
Platanares Loan - OPIC [Member]
Loans	107.0	112.1
Amatitlan Loan [Member]
Loans	22.6	23.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	74.4	78.5
USG Prudential - NV [Member]
Notes	30.2	31.8
USG Prudential - ID [Member]
Notes	17.4	18.3
USG DOE [Member]
Notes	41.7	45.1
Senior Unsecured Bonds [Member]
Senior Unsecured debt	565.8	585.1
Senior Unsecured Loan [Member]
Senior Unsecured debt	215.7	222.2
Plumstriker Loan Agreement [Member]
Loans	17.8	18.1
Estimate of Fair Value Measurement [Member]
Other long-term debt	16.2	17.4
Estimate of Fair Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	184.2	192.5
Estimate of Fair Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	40.6	40.4
Estimate of Fair Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	35.8	35.8
Estimate of Fair Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	107.0	112.1
Estimate of Fair Value Measurement [Member] | Amatitlan Loan [Member]
Loans	22.6	23.5
Estimate of Fair Value Measurement [Member] | OFC Two Senior Secured Notes [Member]
Notes	197.5	207.9
Estimate of Fair Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	74.4	78.5
Estimate of Fair Value Measurement [Member] | USG Prudential - NV [Member]
Notes	30.2	31.8
Estimate of Fair Value Measurement [Member] | USG Prudential - ID [Member]
Notes	17.4	18.3
Estimate of Fair Value Measurement [Member] | USG DOE [Member]
Notes	41.7	45.1
Estimate of Fair Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	565.8	585.1
Estimate of Fair Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	215.7	222.2
Estimate of Fair Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	17.8	18.1
Reported Value Measurement [Member]
Other long-term debt	16.5	17.6
Reported Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	170.2	174.7
Reported Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	37.5	37.5
Reported Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	32.8	32.8
Reported Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	94.2	96.3
Reported Value Measurement [Member] | Amatitlan Loan [Member]
Loans	21.9	22.8
Reported Value Measurement [Member] | OFC Two Senior Secured Notes [Member]
Notes	183.6	188.2
Reported Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	71.5	73.1
Reported Value Measurement [Member] | USG Prudential - NV [Member]
Notes	27.4	27.6
Reported Value Measurement [Member] | USG Prudential - ID [Member]
Notes	17.6	18.4
Reported Value Measurement [Member] | USG DOE [Member]
Notes	36.8	38.2
Reported Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	518.0	529.1
Reported Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	200.0	200.0
Reported Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	$ 17.8	$ 18.1