Note 5 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 16.1	$ 5.4
Reported Value Measurement [Member]
Other long-term debt	17.1	6.2
Olkaria III OPIC [Member]
Loans	208.0	211.8
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	208.0	211.8
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	197.1	210.6
Olkaria IV Loan - DEG 2 [Member]
Loans	47.3
Olkaria IV Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	47.3	47.2
Olkaria IV Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	45.0	47.5
Olkaria IV Loan - DEG 3 [Member]
Loans	41.8
Olkaria IV Loan - DEG 3 [Member] | Estimate of Fair Value Measurement [Member]
Loans	41.8	0.0
Olkaria IV Loan - DEG 3 [Member] | Reported Value Measurement [Member]
Loans	39.3	0.0
Platanares Loan - OPIC [Member]
Loans	118.6	119.1
Platanares Loan - OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	118.6	119.1
Platanares Loan - OPIC [Member] | Reported Value Measurement [Member]
Loans	106.5	112.7
Amatitlan Loan [Member]
Loans	27.3	29.9
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	27.3	29.9
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	27.1	29.8
OrCal Geothermal Inc [Member]
Notes	15.4	19.0
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	15.4	19.0
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	15.0	18.7
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	216.9	214.5
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	207.1	217.8
Don A. Campbell 1 ("DAC1") [Member]
Notes	80.6	78.8
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	80.6	78.8
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	79.6	83.3
USG Prudential - NV [Member]
Notes	31.4	29.4
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	31.4	29.4
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	27.5	27.8
USG Prudential - ID [Member]
Notes	18.4	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.4	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.3	18.9
USG DOE [Member]
Notes	45.1	48.3
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	45.1	48.3
USG DOE [Member] | Reported Value Measurement [Member]
Notes	44.9	51.4
Senior Unsecured Bonds [Member]
Senior Unsecured debt	203.4	199.4
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	203.4	199.4
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior Unsecured debt	160.9	102.2
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	160.9	102.2
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	150.0	100.0
Plumstriker Loan Agreement [Member]
Loans	23.7
Plumstriker Loan Agreement [Member] | Estimate of Fair Value Measurement [Member]
Loans	23.7	0.0
Plumstriker Loan Agreement [Member] | Reported Value Measurement [Member]
Loans	$ 23.2	$ 0.0