Note 4 - Fair Value of Financial Instruments - Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Estimate of Fair Value Measurement [Member]
Other long-term debt	$ 16.2	$ 16.3
Reported Value Measurement [Member]
Other long-term debt	16.6	17.4
Olkaria III OPIC [Member]
Loans	206.5	202.1
Olkaria III OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	206.5	202.1
Olkaria III OPIC [Member] | Reported Value Measurement [Member]
Loans	188.1	192.6
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	46.2	43.8
Olkaria III Plant 4 Loan - DEG 2 [Member] | Estimate of Fair Value Measurement [Member]
Loans	46.2	43.8
Olkaria III Plant 4 Loan - DEG 2 [Member] | Reported Value Measurement [Member]
Loans	42.5	42.5
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	40.8	38.8
Olkaria III plant 1 Loan - DEG 3 [Member] | Estimate of Fair Value Measurement [Member]
Loans	40.8	38.8
Olkaria III plant 1 Loan - DEG 3 [Member] | Reported Value Measurement [Member]
Loans	37.1	37.1
Platanares Loan - OPIC [Member]
Loans	118.9	115.3
Platanares Loan - OPIC [Member] | Estimate of Fair Value Measurement [Member]
Loans	118.9	115.3
Platanares Loan - OPIC [Member] | Reported Value Measurement [Member]
Loans	102.4	104.5
Amatitlan Loan [Member]
Loans	25.5	26.4
Amatitlan Loan [Member] | Estimate of Fair Value Measurement [Member]
Loans	25.5	26.4
Amatitlan Loan [Member] | Reported Value Measurement [Member]
Loans	25.4	26.3
OFC Two Senior Secured Notes [Member] | Estimate of Fair Value Measurement [Member]
Notes	218.1	210.9
OFC Two Senior Secured Notes [Member] | Reported Value Measurement [Member]
Notes	198.4	203.0
Don A. Campbell 1 ("DAC1") [Member]
Notes	81.7	78.5
Don A. Campbell 1 ("DAC1") [Member] | Estimate of Fair Value Measurement [Member]
Notes	81.7	78.5
Don A. Campbell 1 ("DAC1") [Member] | Reported Value Measurement [Member]
Notes	76.6	78.2
USG Prudential - NV [Member]
Notes	32.8	30.6
USG Prudential - NV [Member] | Estimate of Fair Value Measurement [Member]
Notes	32.8	30.6
USG Prudential - NV [Member] | Reported Value Measurement [Member]
Notes	28.3	28.4
USG Prudential - ID [Member]
Notes	18.3	18.6
USG Prudential - ID [Member] | Estimate of Fair Value Measurement [Member]
Notes	18.3	18.6
USG Prudential - ID [Member] | Reported Value Measurement [Member]
Notes	18.9	19.6
USG DOE [Member]
Notes	46.2	45.0
USG DOE [Member] | Estimate of Fair Value Measurement [Member]
Notes	46.2	45.0
USG DOE [Member] | Reported Value Measurement [Member]
Notes	39.4	40.8
Senior Unsecured Bonds [Member]
Senior Unsecured debt	207.7	205.7
Senior Unsecured Bonds [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	207.7	205.7
Senior Unsecured Bonds [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	204.3	204.3
Senior Unsecured Loan [Member]
Senior Unsecured debt	169.0	161.3
Senior Unsecured Loan [Member] | Estimate of Fair Value Measurement [Member]
Senior Unsecured debt	169.0	161.3
Senior Unsecured Loan [Member] | Reported Value Measurement [Member]
Senior Unsecured debt	150.0	150.0
Plumstriker Loan Agreement [Member]
Loans	21.4
Senior Unsecured debt		21.7
Plumstriker Loan Agreement [Member] | Estimate of Fair Value Measurement [Member]
Loans	21.4	21.7
Plumstriker Loan Agreement [Member] | Reported Value Measurement [Member]
Loans	$ 21.3	$ 21.6