Fair Value of Long-term Debt Approximates Its Carrying Amount Except for the Following (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans from institutional investors	$ 34.2	$ 37.1
Fair Value | Olkaria III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	83.2	88.7
Fair Value | Amatitlan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	37.8	39.5
Fair Value | Ormat Funding Corp.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	129.5	129.5
Fair Value | OrCal Geothermal Inc.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	92.8	93.5
Fair Value | Senior Unsecured Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Bonds	252.8	144.8
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans from institutional investors	34.2	37.2
Carrying Amount | Olkaria III
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	82.9	88.4
Carrying Amount | Amatitlan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loan	37.3	39.0
Carrying Amount | Ormat Funding Corp.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	130.8	136.3
Carrying Amount | OrCal Geothermal Inc.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior secured notes	93.2	95.6
Carrying Amount | Senior Unsecured Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Bonds	$ 248.3	$ 142.0