Fair Value of Long-term Debt Approximates Its Carrying Amount, Exceptions (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	$ 50.0
Olkaria III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	80.3
Amatitlan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	36.6
Ormat Funding Corp. ("OFC")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	118.7
OrCal Geothermal Inc. ("OrCal")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	85.8
OFC 2 LLC ("OFC 2")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	125.0
Senior Unsecured Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Bonds	238.7
Institutional Investors
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	32.5
Fair Value | Olkaria III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	80.3	79.2
Fair Value | Amatitlan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	36.6	37.2
Fair Value | Ormat Funding Corp. ("OFC")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	118.7	114.8
Fair Value | OrCal Geothermal Inc. ("OrCal")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	85.8	84.4
Fair Value | OFC 2 LLC ("OFC 2")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	125.0	131.0
Fair Value | Senior Unsecured Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Bonds	238.7	252.8
Fair Value | Institutional Investors
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	32.5	34.2
Carrying Amount | Olkaria III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	77.4	77.4
Carrying Amount | Amatitlan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	36.2	36.8
Carrying Amount | Ormat Funding Corp. ("OFC")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	125.0	125.0
Carrying Amount | OrCal Geothermal Inc. ("OrCal")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	85.9	85.9
Carrying Amount | OFC 2 LLC ("OFC 2")
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	151.7	151.7
Carrying Amount | Senior Unsecured Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Bonds	248.3	248.3
Carrying Amount | Institutional Investors
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other long-term debt	$ 32.6	$ 34.2