Note 4 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Revolving lines of credit	$ 270.5	$ 40.6
Deposits	11.8	12.2
Commercial Paper [Member]
Commercial paper	8.3	50.0
Olkaria III OPIC [Member]
Loans	206.5	202.1
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	46.2	43.8
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	40.8	38.8
Platanares Loan - OPIC [Member]
Loans	118.9	115.3
Amatitlan Loan [Member]
Loans	25.5	26.4
OFC Senior Secured Notes [Member]
Notes	218.1	210.9
Don A. Campbell 1 ("DAC1") [Member]
Notes	81.7	78.5
USG Prudential - NV [Member]
Notes	32.8	30.6
USG Prudential - ID [Member]
Notes	18.3	18.6
USG DOE [Member]
Notes	46.2	45.0
Senior Unsecured Bonds [Member]
Senior Unsecured debt	207.7	205.7
Senior Unsecured Loan [Member]
Senior Unsecured debt	169.0	161.3
Plumstriker Loan Agreement [Member]
Loans	21.4
Senior Unsecured debt		21.7
Other Long-term Debt [Member]
Senior Unsecured debt	16.2	16.3
Fair Value, Inputs, Level 1 [Member]
Revolving lines of credit	0.0	0.0
Deposits	11.8	12.2
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Commercial paper	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III OPIC [Member]
Loans	0.0	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] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0	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
Senior Unsecured debt		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]
Revolving lines of credit	270.5	40.6
Deposits	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Commercial paper	8.3	50.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III OPIC [Member]
Loans	0.0	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] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0	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	25.5	26.4
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	21.4
Senior Unsecured debt		21.7
Fair Value, Inputs, Level 2 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 3 [Member]
Revolving lines of credit	0.0	0.0
Deposits	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Commercial paper	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Olkaria III OPIC [Member]
Loans	206.5	202.1
Fair Value, Inputs, Level 3 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	46.2	43.8
Fair Value, Inputs, Level 3 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	40.8	38.8
Fair Value, Inputs, Level 3 [Member] | Platanares Loan - OPIC [Member]
Loans	118.9	115.3
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	218.1	210.9
Fair Value, Inputs, Level 3 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	81.7	78.5
Fair Value, Inputs, Level 3 [Member] | USG Prudential - NV [Member]
Notes	32.8	30.6
Fair Value, Inputs, Level 3 [Member] | USG Prudential - ID [Member]
Notes	18.3	18.6
Fair Value, Inputs, Level 3 [Member] | USG DOE [Member]
Notes	46.2	45.0
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	207.7	205.7
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	169.0	161.3
Fair Value, Inputs, Level 3 [Member] | Plumstriker Loan Agreement [Member]
Loans	0.0
Senior Unsecured debt		0.0
Fair Value, Inputs, Level 3 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	$ 16.2	$ 16.3