Note 7 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 16.3	$ 5.4
Reported Value Measurement [Member]
Other long-term debt	17.4	6.2
Olkaria III OPIC [Member]
Loans	202.1	211.8
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	202.1	211.8
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	192.6	210.6
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	43.8
Olkaria III Plant 4 Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	43.8	47.2
Olkaria III Plant 4 Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	42.5	47.5
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	38.8
Olkaria III plant 1 Loan - DEG 3 [Member] | Estimate of Fair Value Measurement [Member]
Loans	38.8	0.0
Olkaria III plant 1 Loan - DEG 3 [Member] | Reported Value Measurement [Member]
Loans	37.1	0.0
Platanares Loan - OPIC [Member]
Loans	115.3	119.1
Platanares Loan - OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	115.3	119.1
Platanares Loan - OPIC [Member] | Reported Value Measurement [Member]
Loans	104.5	112.7
Amatitlan Loan [Member]
Loans	26.4	29.9
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	26.4	29.9
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	26.3	29.8
OrCal Geothermal Inc [Member]
Notes		19.0
OrCal Geothermal Inc [Member] | Estimate of Fair Value Measurement [Member]
Notes	0.0	19.0
OrCal Geothermal Inc [Member] | Reported Value Measurement [Member]
Notes	0.0	18.7
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	210.9	214.5
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	203.0	217.8
Don A. Campbell 1 ("DAC1") [Member]
Notes	78.5	78.8
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	78.5	78.8
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	78.2	83.3
USG Prudential - NV [Member]
Notes	30.6	29.4
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	30.6	29.4
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	28.4	27.8
USG Prudential - ID [Member]
Notes	18.6	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.6	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	19.6	18.9
USG DOE [Member]
Notes	45.0	48.3
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	45.0	48.3
USG DOE [Member] | Reported Value Measurement [Member]
Notes	40.8	51.4
Senior Unsecured Bonds [Member]
Senior Unsecured debt	205.7	199.4
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	205.7	199.4
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior Unsecured debt	161.3	102.2
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	161.3	102.2
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	150.0	100.0
Plumstriker Loan Agreement [Member]
Loans	21.7
Plumstriker Loan Agreement [Member] | Estimate of Fair Value Measurement [Member]
Loans	21.7	0.0
Plumstriker Loan Agreement [Member] | Reported Value Measurement [Member]
Loans	$ 21.6	$ 0.0