Note 5 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Revolving lines of credit	$ 158.6	$ 51.5
Deposits	19.3	15.6
Fair Value, Inputs, Level 1 [Member]
Revolving lines of credit
Deposits	19.3	15.6
Fair Value, Inputs, Level 2 [Member]
Revolving lines of credit	158.6	51.5
Deposits
Fair Value, Inputs, Level 3 [Member]
Revolving lines of credit
Deposits
Olkaria III OPIC [Member]
Loans	219.6	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	219.6	234.6
Olkaria IV Loan - DEG 2 [Member]
Loans	49.5
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	49.5	50.7
Amatitlan Loan [Member]
Loans	31.7	32.8
Amatitlan Loan [Member] | Fair Value, Inputs, Level 1 [Member]
Loans
Amatitlan Loan [Member] | Fair Value, Inputs, Level 2 [Member]
Loans	31.7	32.8
Amatitlan Loan [Member] | Fair Value, Inputs, Level 3 [Member]
Loans
OrCal Geothermal Inc [Member]
Notes	28.7	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	28.7	34.2
OFC Two Senior Secured Notes [Member]
Notes	219.2	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	219.2	234.6
Don A. Campbell 1 ("DAC1") [Member]
Notes	79.9	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	79.9	85.5
USG Prudential - NV [Member]
Notes	29.7
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	29.7
Senior Unsecured Bonds [Member]
Senior secured debt	198.1	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	198.1	200.3
USG Prudential - ID [Member]
Notes	18.6
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	18.6
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
USG DOE [Member]
Notes	49.0
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	49.0
Senior Unsecured Loan [Member]
Senior secured debt	101.3
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	$ 101.3