Note 5 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Revolving lines of credit	$ 209.5	$ 51.5
Deposits	14.2	15.6
Fair Value, Inputs, Level 1 [Member]
Revolving lines of credit
Deposits	14.2	15.6
Fair Value, Inputs, Level 2 [Member]
Revolving lines of credit	209.5	51.5
Deposits
Fair Value, Inputs, Level 3 [Member]
Revolving lines of credit
Deposits
Olkaria III OPIC [Member]
Loans	214.2	234.6
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 1 [Member]
Loans
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 2 [Member]
Loans
Olkaria III OPIC [Member] | Fair Value, Inputs, Level 3 [Member]
Loans	214.2	234.6
Olkaria IV Loan - DEG 2 [Member]
Loans	50.1
Olkaria IV Loan - DEG 2 [Member] | Fair Value, Inputs, Level 1 [Member]
Loans
Olkaria IV Loan - DEG 2 [Member] | Fair Value, Inputs, Level 2 [Member]
Loans
Olkaria IV Loan - DEG 2 [Member] | Fair Value, Inputs, Level 3 [Member]
Loans	50.1	50.7
Amatitlan Loan [Member]
Loans	30.8	32.8
Amatitlan Loan [Member] | Fair Value, Inputs, Level 1 [Member]
Loans
Amatitlan Loan [Member] | Fair Value, Inputs, Level 2 [Member]
Loans	30.8	32.8
Amatitlan Loan [Member] | Fair Value, Inputs, Level 3 [Member]
Loans
OrCal Geothermal Inc [Member]
Notes	24.7	34.2
OrCal Geothermal Inc [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
OrCal Geothermal Inc [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
OrCal Geothermal Inc [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	24.7	34.2
OFC Senior Secured Notes [Member]
Notes
OFC Senior Secured Notes [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
OFC Senior Secured Notes [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
OFC Senior Secured Notes [Member] | Fair Value, Inputs, Level 3 [Member]
Notes
OFC Two Senior Secured Notes [Member]
Notes	215.9	234.6
OFC Two Senior Secured Notes [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
OFC Two Senior Secured Notes [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
OFC Two Senior Secured Notes [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	215.9	234.6
Don A. Campbell 1 ("DAC1") [Member]
Notes	79.0	85.5
Don A. Campbell 1 ("DAC1") [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
Don A. Campbell 1 ("DAC1") [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
Don A. Campbell 1 ("DAC1") [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	79.0	85.5
USG Prudential - NV [Member]
Notes	29.4
USG Prudential - NV [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
USG Prudential - NV [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
USG Prudential - NV [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	29.4
USG Prudential - ID [Member]
Notes	18.7
USG Prudential - ID [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
USG Prudential - ID [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
USG Prudential - ID [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	18.7
Senior Unsecured Bonds [Member]
Senior secured debt	195.7	200.3
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Senior secured debt
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Senior secured debt
Senior Unsecured Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Senior secured debt	195.7	200.3
USG DOE [Member]
Notes	47.2
USG DOE [Member] | Fair Value, Inputs, Level 1 [Member]
Notes
USG DOE [Member] | Fair Value, Inputs, Level 2 [Member]
Notes
USG DOE [Member] | Fair Value, Inputs, Level 3 [Member]
Notes	47.2
Other Long-term Debt [Member]
Senior secured debt		7.0
Other long-term debt	5.4
Other Long-term Debt [Member] | Fair Value, Inputs, Level 1 [Member]
Senior secured debt
Other long-term debt
Other Long-term Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Senior secured debt
Other long-term debt
Other Long-term Debt [Member] | Fair Value, Inputs, Level 3 [Member]
Senior secured debt		$ 7.0
Other long-term debt	5.4
Senior Unsecured Loan [Member]
Senior secured debt	99.7
Senior Unsecured Loan [Member] | Fair Value, Inputs, Level 1 [Member]
Senior secured debt
Senior Unsecured Loan [Member] | Fair Value, Inputs, Level 2 [Member]
Senior secured debt
Senior Unsecured Loan [Member] | Fair Value, Inputs, Level 3 [Member]
Senior secured debt	$ 99.7