Note 5 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Finance liability - Dixie Valley	$ 252.2
Deposits	17.1	$ 14.8
HSBC Loan Agreement [Member]
Loans	57.0
Olkaria III Loan DFC [Member]
Loans	173.9	192.5
Hapoalim Loan Agreement [Member]
Loans	128.6
Olkaria IV Loan - DEG 2 [Member]
Loans		40.4
Discount Loan Agreement [Member]
Loans	101.8
Olkaria IV Loan - DEG 3 [Member]
Loans		35.8
Platanares Loan - OPIC [Member]
Loans	102.2	112.1
Amatitlan Loan [Member]
Loans	20.8	23.5
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	37.6
OFC Senior Secured Notes [Member]
Notes	190.8	207.9
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	33.2
Don A. Campbell 1 ("DAC1") [Member]
Notes	72.4	78.5
USG Prudential - NV [Member]
Notes	29.8	31.8
USG Prudential - ID [Member]
Notes	17.1	18.3
USG DOE [Member]
Notes	40.5	45.1
Senior Unsecured Bonds [Member]
Senior Unsecured debt	572.1	585.1
Senior Unsecured Loan [Member]
Senior Unsecured debt	206.2	222.2
Plumstriker Loan Agreement [Member]
Loans	16.3	18.1
Other Long-term Debt [Member]
Senior Unsecured debt	14.5	17.4
Fair Value, Inputs, Level 1 [Member]
Finance liability - Dixie Valley	0.0
Deposits	17.1	14.8
Fair Value, Inputs, Level 1 [Member] | HSBC Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III Loan DFC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Hapoalim Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans		0.0
Fair Value, Inputs, Level 1 [Member] | Discount Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans		0.0
Fair Value, Inputs, Level 1 [Member] | Platanares Loan - OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Amatitlan Loan [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | OFC Senior Secured Notes [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG Prudential - NV [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG Prudential - ID [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG DOE [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Plumstriker Loan Agreement [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Finance liability - Dixie Valley	0.0
Deposits	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | HSBC Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III Loan DFC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Hapoalim Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans		0.0
Fair Value, Inputs, Level 2 [Member] | Discount Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans		0.0
Fair Value, Inputs, Level 2 [Member] | Platanares Loan - OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Amatitlan Loan [Member]
Loans	20.8	23.5
Fair Value, Inputs, Level 2 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | OFC Senior Secured Notes [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG Prudential - NV [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG Prudential - ID [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG DOE [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Plumstriker Loan Agreement [Member]
Loans	16.3	18.1
Fair Value, Inputs, Level 2 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 3 [Member]
Finance liability - Dixie Valley	252.2
Deposits	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | HSBC Loan Agreement [Member]
Loans	57.0
Fair Value, Inputs, Level 3 [Member] | Olkaria III Loan DFC [Member]
Loans	173.9	192.5
Fair Value, Inputs, Level 3 [Member] | Hapoalim Loan Agreement [Member]
Loans	128.6
Fair Value, Inputs, Level 3 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans		40.4
Fair Value, Inputs, Level 3 [Member] | Discount Loan Agreement [Member]
Loans	101.8
Fair Value, Inputs, Level 3 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans		35.8
Fair Value, Inputs, Level 3 [Member] | Platanares Loan - OPIC [Member]
Loans	102.2	112.1
Fair Value, Inputs, Level 3 [Member] | Amatitlan Loan [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	37.6
Fair Value, Inputs, Level 3 [Member] | OFC Senior Secured Notes [Member]
Notes	190.8	207.9
Fair Value, Inputs, Level 3 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	33.2
Fair Value, Inputs, Level 3 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	72.4	78.5
Fair Value, Inputs, Level 3 [Member] | USG Prudential - NV [Member]
Notes	29.8	31.8
Fair Value, Inputs, Level 3 [Member] | USG Prudential - ID [Member]
Notes	17.1	18.3
Fair Value, Inputs, Level 3 [Member] | USG DOE [Member]
Notes	40.5	45.1
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	572.1	585.1
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	206.2	222.2
Fair Value, Inputs, Level 3 [Member] | Plumstriker Loan Agreement [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	$ 14.5	$ 17.4