Note 8 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deposits	$ 17.1	$ 14.8
HSBC Loan Agreement [Member]
Loans	50.4
Olkaria III Loan DFC [Member]
Loans	166.5	192.5
Hapoalim Loan Agreement [Member]
Loans	117.8
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.1	40.4
Discount Loan Agreement [Member]
Loans	100.2
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.1	35.8
Dixie Valley Loan [Member]
Loans	248.4
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
Other Long-term Debt [Member]
Senior Unsecured debt	13.3	17.4
Fair Value, Inputs, Level 1 [Member]
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 III Plant 4 Loan - DEG 2 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Discount Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Dixie Valley Loan [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] | OFC Senior Secured Notes [Member]
Notes	0.0	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]
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 III Plant 4 Loan - DEG 2 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Discount Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Dixie Valley Loan [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	19.8	23.5
Fair Value, Inputs, Level 2 [Member] | OFC Senior Secured Notes [Member]
Notes	0.0	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	14.8	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]
Deposits	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | HSBC Loan Agreement [Member]
Loans	50.4
Fair Value, Inputs, Level 3 [Member] | Olkaria III Loan DFC [Member]
Loans	166.5	192.5
Fair Value, Inputs, Level 3 [Member] | Hapoalim Loan Agreement [Member]
Loans	117.8
Fair Value, Inputs, Level 3 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	34.1	40.4
Fair Value, Inputs, Level 3 [Member] | Discount Loan Agreement [Member]
Loans	100.2
Fair Value, Inputs, Level 3 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	30.1	35.8
Fair Value, Inputs, Level 3 [Member] | Dixie Valley Loan [Member]
Loans	248.4
Fair Value, Inputs, Level 3 [Member] | Platanares Loan - OPIC [Member]
Loans	98.2	112.1
Fair Value, Inputs, Level 3 [Member] | Amatitlan Loan [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | OFC Senior Secured Notes [Member]
Notes	183.3	207.9
Fair Value, Inputs, Level 3 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	69.8	78.5
Fair Value, Inputs, Level 3 [Member] | USG Prudential - NV [Member]
Notes	28.9	31.8
Fair Value, Inputs, Level 3 [Member] | USG Prudential - ID [Member]
Notes	17.3	18.3
Fair Value, Inputs, Level 3 [Member] | USG DOE [Member]
Notes	39.9	45.1
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	578.9	585.1
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	204.3	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	$ 13.3	$ 17.4