Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 5.4	$ 7.0
Reported Value Measurement [Member]
Other long-term debt	6.4	7.9
Olkaria III OPIC [Member]
Loans	219.6	234.6
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	219.6	234.6
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	219.6	228.6
Olkaria IV Loan - DEG 2 [Member]
Loans	49.5
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	49.5	50.7
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	50.0	50.0
Amatitlan Loan [Member]
Loans	31.7	32.8
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	31.7	32.8
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	31.5	33.3
OrCal Geothermal Inc [Member]
Notes	28.7	34.2
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	28.7	34.2
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	27.3	32.1
OFC Two Senior Secured Notes [Member]
Notes	219.2	234.6
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	219.2	234.6
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	224.2	232.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	79.9	85.5
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	79.9	85.5
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	85.3	88.3
USG Prudential - NV [Member]
Notes	29.7
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	29.7
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	28.3
USG Prudential - ID [Member]
Notes	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.9
USG DOE [Member]
Notes	49.0
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	49.0
USG DOE [Member] | Reported Value Measurement [Member]
Notes	53.0
Senior Unsecured Bonds [Member]
Senior secured debt	198.1	200.3
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior secured debt	198.1	200.3
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior secured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior secured debt	101.3
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior secured debt	101.3
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior secured debt	$ 100.0