Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Finance liability - Dixie Valley	$ 245.2
HSBC Loan Agreement [Member]
Loans	46.8
Hapoalim Loan Agreement [Member]
Loans	117.8
Discount Loan Agreement [Member]
Loans	94.0
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.4
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.3
Platanares Loan - OPIC [Member]
Loans	95.6	$ 98.2
Amatitlan Loan [Member]
Loans	18.6	19.8
Don A. Campbell 1 ("DAC1") [Member]
Notes	68.0	69.8
USG Prudential - NV [Member]
Notes	28.6	28.9
USG Prudential - ID [Member]
Notes	16.2	17.3
USG DOE [Member]
Notes	38.3	39.9
Senior Unsecured Bonds [Member]
Senior Unsecured debt	569.5	578.9
Senior Unsecured Loan [Member]
Senior Unsecured debt	193.5	204.3
Plumstriker Loan Agreement [Member]
Loans	14.5	14.8
Estimate of Fair Value Measurement [Member]
Finance liability - Dixie Valley	245.2	248.4
Other long-term debt	12.1	13.3
Estimate of Fair Value Measurement [Member] | HSBC Loan Agreement [Member]
Loans	46.8	50.4
Estimate of Fair Value Measurement [Member] | Hapoalim Loan Agreement [Member]
Loans	117.8	117.8
Estimate of Fair Value Measurement [Member] | Discount Loan Agreement [Member]
Loans	94.0	100.2
Estimate of Fair Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	161.2	166.5
Estimate of Fair Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.4	34.1
Estimate of Fair Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.3	30.1
Estimate of Fair Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	95.6	98.2
Estimate of Fair Value Measurement [Member] | Amatitlan Loan [Member]
Loans	18.6	19.8
Estimate of Fair Value Measurement [Member] | OFC Two Senior Secured Notes [Member]
Notes	178.1	183.3
Estimate of Fair Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	68.0	69.8
Estimate of Fair Value Measurement [Member] | USG Prudential - NV [Member]
Notes	28.6	28.9
Estimate of Fair Value Measurement [Member] | USG Prudential - ID [Member]
Notes	16.2	17.3
Estimate of Fair Value Measurement [Member] | USG DOE [Member]
Notes	38.3	39.9
Estimate of Fair Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	569.5	578.9
Estimate of Fair Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	193.5	204.3
Estimate of Fair Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	14.5	14.8
Reported Value Measurement [Member]
Finance liability - Dixie Valley	249.1	252.9
Other long-term debt	12.6	13.6
Reported Value Measurement [Member] | HSBC Loan Agreement [Member]
Loans	46.4	50.0
Reported Value Measurement [Member] | Hapoalim Loan Agreement [Member]
Loans	116.1	116.1
Reported Value Measurement [Member] | Discount Loan Agreement [Member]
Loans	93.8	100.0
Reported Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	152.2	156.7
Reported Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	32.5	32.5
Reported Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	28.4	28.4
Reported Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	86.0	88.1
Reported Value Measurement [Member] | Amatitlan Loan [Member]
Loans	18.4	19.3
Reported Value Measurement [Member] | OFC Two Senior Secured Notes [Member]
Notes	168.8	173.3
Reported Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	66.3	67.9
Reported Value Measurement [Member] | USG Prudential - NV [Member]
Notes	26.1	26.3
Reported Value Measurement [Member] | USG Prudential - ID [Member]
Notes	16.5	17.3
Reported Value Measurement [Member] | USG DOE [Member]
Notes	34.1	35.5
Reported Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	532.9	539.6
Reported Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	183.2	191.6
Reported Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	$ 14.4	$ 14.7