Note 5 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Revolving lines of credit	$ 60.9	$ 159.0
Deposits	11.9	12.0
Olkaria III OPIC [Member]
Loans	210.0	211.8
Olkaria IV Loan - DEG 2 [Member]
Loans	48.6
Olkaria IV Loan - DEG 3 [Member]
Loans	42.5
Platanares Loan - OPIC [Member]
Loans	118.5	119.1
Amatitlan Loan [Member]
Loans	29.0	29.9
OrCal Geothermal Inc [Member]
Notes	19.2	19.0
OFC Senior Secured Notes [Member]
Notes	213.4	214.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	78.6	78.8
USG Prudential - NV [Member]
Notes	29.9	29.4
USG Prudential - ID [Member]
Notes	18.0	18.6
USG DOE [Member]
Notes	47.3	48.3
Senior Unsecured Bonds [Member]
Senior Unsecured debt	199.3	199.4
Senior Unsecured Loan [Member]
Senior Unsecured debt	153.6	102.2
Other Long-term Debt [Member]
Senior Unsecured debt	5.3	5.4
Fair Value, Inputs, Level 1 [Member]
Revolving lines of credit
Deposits	11.9	12.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III OPIC [Member]
Loans
Fair Value, Inputs, Level 1 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans
Fair Value, Inputs, Level 1 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans
Fair Value, Inputs, Level 1 [Member] | Platanares Loan - OPIC [Member]
Loans
Fair Value, Inputs, Level 1 [Member] | Amatitlan Loan [Member]
Loans
Fair Value, Inputs, Level 1 [Member] | OrCal Geothermal Inc [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | OFC Senior Secured Notes [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | USG Prudential - NV [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | USG Prudential - ID [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | USG DOE [Member]
Notes
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 1 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 2 [Member]
Revolving lines of credit	60.9	159.0
Deposits
Fair Value, Inputs, Level 2 [Member] | Olkaria III OPIC [Member]
Loans
Fair Value, Inputs, Level 2 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans
Fair Value, Inputs, Level 2 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans
Fair Value, Inputs, Level 2 [Member] | Platanares Loan - OPIC [Member]
Loans
Fair Value, Inputs, Level 2 [Member] | Amatitlan Loan [Member]
Loans	29.0	29.9
Fair Value, Inputs, Level 2 [Member] | OrCal Geothermal Inc [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | OFC Senior Secured Notes [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | USG Prudential - NV [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | USG Prudential - ID [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | USG DOE [Member]
Notes
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 2 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt
Fair Value, Inputs, Level 3 [Member]
Revolving lines of credit
Deposits
Fair Value, Inputs, Level 3 [Member] | Olkaria III OPIC [Member]
Loans	210.0	211.8
Fair Value, Inputs, Level 3 [Member] | Olkaria IV Loan - DEG 2 [Member]
Loans	48.6	47.2
Fair Value, Inputs, Level 3 [Member] | Olkaria IV Loan - DEG 3 [Member]
Loans	42.5
Fair Value, Inputs, Level 3 [Member] | Platanares Loan - OPIC [Member]
Loans	118.5	119.1
Fair Value, Inputs, Level 3 [Member] | Amatitlan Loan [Member]
Loans
Fair Value, Inputs, Level 3 [Member] | OrCal Geothermal Inc [Member]
Notes	19.2	19.0
Fair Value, Inputs, Level 3 [Member] | OFC Senior Secured Notes [Member]
Notes	213.4	214.5
Fair Value, Inputs, Level 3 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	78.6	78.8
Fair Value, Inputs, Level 3 [Member] | USG Prudential - NV [Member]
Notes	29.9	29.4
Fair Value, Inputs, Level 3 [Member] | USG Prudential - ID [Member]
Notes	18.0	18.6
Fair Value, Inputs, Level 3 [Member] | USG DOE [Member]
Notes	47.3	48.3
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	199.3	199.4
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	153.6	102.2
Fair Value, Inputs, Level 3 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	$ 5.3	$ 5.4