Note 7 - Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Revolving lines of credit	$ 40.6	$ 159.0
Deposits	12.2	12.0
Commercial Paper [Member]
Commercial paper	50.0
Olkaria III OPIC [Member]
Loans	202.1	211.8
Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	43.8
Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	38.8
Platanares Loan - OPIC [Member]
Loans	115.3	119.1
Amatitlan Loan [Member]
Loans	26.4	29.9
OrCal Geothermal Inc [Member]
Notes		19.0
OFC Senior Secured Notes [Member]
Notes	210.9	214.5
Don A. Campbell 1 ("DAC1") [Member]
Notes	78.5	78.8
USG Prudential - NV [Member]
Notes	30.6	29.4
USG Prudential - ID [Member]
Notes	18.6	18.6
USG DOE [Member]
Notes	45.0	48.3
Senior Unsecured Bonds [Member]
Senior Unsecured debt	205.7	199.4
Senior Unsecured Loan [Member]
Senior Unsecured debt	161.3	102.2
Plumstriker Loan Agreement [Member]
Loans	21.7
Other Long-term Debt [Member]
Senior Unsecured debt	16.3	5.4
Fair Value, Inputs, Level 1 [Member]
Revolving lines of credit	0.0	0.0
Deposits	12.2	12.0
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Commercial paper	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Platanares Loan - OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Amatitlan Loan [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | OrCal Geothermal Inc [Member]
Notes		0.0
Fair Value, Inputs, Level 1 [Member] | OFC Senior Secured Notes [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG Prudential - NV [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG Prudential - ID [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | USG DOE [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Plumstriker Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 1 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Revolving lines of credit	40.6	159.0
Deposits	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Commercial paper	50.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	0.0
Fair Value, Inputs, Level 2 [Member] | Platanares Loan - OPIC [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Amatitlan Loan [Member]
Loans	26.4	29.9
Fair Value, Inputs, Level 2 [Member] | OrCal Geothermal Inc [Member]
Notes		0.0
Fair Value, Inputs, Level 2 [Member] | OFC Senior Secured Notes [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG Prudential - NV [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG Prudential - ID [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | USG DOE [Member]
Notes	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Plumstriker Loan Agreement [Member]
Loans	21.7
Fair Value, Inputs, Level 2 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	0.0	0.0
Fair Value, Inputs, Level 3 [Member]
Revolving lines of credit	0.0	0.0
Deposits	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Commercial paper	0.0
Fair Value, Inputs, Level 3 [Member] | Olkaria III OPIC [Member]
Loans	202.1	211.8
Fair Value, Inputs, Level 3 [Member] | Olkaria III Plant 4 Loan - DEG 2 [Member]
Loans	43.8	47.2
Fair Value, Inputs, Level 3 [Member] | Olkaria III plant 1 Loan - DEG 3 [Member]
Loans	38.8
Fair Value, Inputs, Level 3 [Member] | Platanares Loan - OPIC [Member]
Loans	115.3	119.1
Fair Value, Inputs, Level 3 [Member] | Amatitlan Loan [Member]
Loans	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | OrCal Geothermal Inc [Member]
Notes		19.0
Fair Value, Inputs, Level 3 [Member] | OFC Senior Secured Notes [Member]
Notes	210.9	214.5
Fair Value, Inputs, Level 3 [Member] | Don A. Campbell 1 ("DAC1") [Member]
Notes	78.5	78.8
Fair Value, Inputs, Level 3 [Member] | USG Prudential - NV [Member]
Notes	30.6	29.4
Fair Value, Inputs, Level 3 [Member] | USG Prudential - ID [Member]
Notes	18.6	18.6
Fair Value, Inputs, Level 3 [Member] | USG DOE [Member]
Notes	45.0	48.3
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Bonds [Member]
Senior Unsecured debt	205.7	199.4
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Loan [Member]
Senior Unsecured debt	161.3	102.2
Fair Value, Inputs, Level 3 [Member] | Plumstriker Loan Agreement [Member]
Loans	0.0
Fair Value, Inputs, Level 3 [Member] | Other Long-term Debt [Member]
Senior Unsecured debt	$ 16.3	$ 5.4