Note 8 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
HSBC Loan Agreement [Member]
Loans	$ 50.4
Hapoalim Loan Agreement [Member]
Loans	117.8
Discount Loan Agreement [Member]
Loans	100.2
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.1	$ 40.4
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.1	35.8
Platanares Loan - OPIC [Member]
Loans	98.2	112.1
Amatitlan Loan [Member]
Loans	19.8	23.5
OFC Senior Secured Notes [Member]
Notes	183.3	207.9
Don A. Campbell 1 ("DAC1") [Member]
Notes	69.8	78.5
USG Prudential - NV [Member]
Notes	28.9	31.8
USG Prudential - ID [Member]
Notes	17.3	18.3
USG DOE [Member]
Notes	39.9	45.1
Senior Unsecured Bonds [Member]
Senior Unsecured debt	578.9	585.1
Senior Unsecured Loan [Member]
Senior Unsecured debt	204.3	222.2
Plumstriker Loan Agreement [Member]
Loans	14.8	18.1
Estimate of Fair Value Measurement [Member]
Financing Liability - Dixie Valley	248.4	0.0
Other long-term debt	13.3	17.4
Estimate of Fair Value Measurement [Member] | HSBC Loan Agreement [Member]
Loans	50.4	0.0
Estimate of Fair Value Measurement [Member] | Hapoalim Loan Agreement [Member]
Loans	117.8	0.0
Estimate of Fair Value Measurement [Member] | Discount Loan Agreement [Member]
Loans	100.2	0.0
Estimate of Fair Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	166.5	192.5
Estimate of Fair Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.1	40.4
Estimate of Fair Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.1	35.8
Estimate of Fair Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	98.2	112.1
Estimate of Fair Value Measurement [Member] | Amatitlan Loan [Member]
Loans	19.8	23.5
Estimate of Fair Value Measurement [Member] | OFC Senior Secured Notes [Member]
Notes	183.3	207.9
Estimate of Fair Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	69.8	78.5
Estimate of Fair Value Measurement [Member] | USG Prudential - NV [Member]
Notes	28.9	31.8
Estimate of Fair Value Measurement [Member] | USG Prudential - ID [Member]
Notes	17.3	18.3
Estimate of Fair Value Measurement [Member] | USG DOE [Member]
Notes	39.9	45.1
Estimate of Fair Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	578.9	585.1
Estimate of Fair Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	204.3	222.2
Estimate of Fair Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	14.8	18.1
Reported Value Measurement [Member]
Financing Liability - Dixie Valley	252.9	0.0
Other long-term debt	13.6	17.6
Reported Value Measurement [Member] | HSBC Loan Agreement [Member]
Loans	50.0	0.0
Reported Value Measurement [Member] | Hapoalim Loan Agreement [Member]
Loans	116.1	0.0
Reported Value Measurement [Member] | Discount Loan Agreement [Member]
Loans	100.0	0.0
Reported Value Measurement [Member] | Olkaria III OPIC [Member]
Loans	156.7	174.7
Reported Value Measurement [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	32.5	37.5
Reported Value Measurement [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	28.4	32.8
Reported Value Measurement [Member] | Platanares Loan - OPIC [Member]
Loans	88.1	96.3
Reported Value Measurement [Member] | Amatitlan Loan [Member]
Loans	19.3	22.8
Reported Value Measurement [Member] | OFC Senior Secured Notes [Member]
Notes	173.3	188.2
Reported Value Measurement [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	67.9	73.1
Reported Value Measurement [Member] | USG Prudential - NV [Member]
Notes	26.3	27.6
Reported Value Measurement [Member] | USG Prudential - ID [Member]
Notes	17.3	18.4
Reported Value Measurement [Member] | USG DOE [Member]
Notes	35.5	38.2
Reported Value Measurement [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	539.6	529.1
Reported Value Measurement [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	191.6	200.0
Reported Value Measurement [Member] | Plumstriker Loan Agreement [Member]
Loans	$ 14.7	$ 18.1