Note 1 - General and Basis of Presentation (Details Textual)			3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 15, 2017 USD ($)	Dec. 29, 2016 USD ($)	Jun. 30, 2017 USD ($) MWh $ / MWh	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) $ / MWh	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Tax Expense (Benefit)			$ 6,369,000	$ 7,890,000	$ 17,255,000	$ 17,399,000
Goodwill			20,121,000		20,121,000		$ 6,650,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			1,000	2,000	3,000	5,000
Interest Expense			14,540,000	18,401,000	29,463,000	34,424,000
Accumulated Other Comprehensive Income (Loss), Net of Tax			(6,933,000)		(6,933,000)		(7,732,000)
Exploration Abandonment and Impairment Expense			0	863,000	0	1,420,000
Cash, Cash Equivalents, and Short-term Investments			118,390,000	$ 192,556,000	118,390,000	$ 192,556,000	230,214,000	$ 185,919,000
Accounts Receivable, Net, Current			$ 79,587,000		79,587,000		$ 80,807,000
Provision for Doubtful Accounts					$ 0
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage					55.00%		60.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage			16.70%	19.10%	17.80%	21.10%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power And Lighting Co LTD [Member]
Concentration Risk, Percentage			15.40%	17.10%	14.80%	17.20%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]
Concentration Risk, Percentage			8.70%	10.40%	8.90%	11.20%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Hyundai (Sarulla Goethermal Power Project) [Member]
Concentration Risk, Percentage			4.90%	8.60%	6.30%	9.00%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments			$ 31,300,000		$ 31,300,000		$ 72,500,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments			99,900,000		99,900,000		166,200,000
Accounts Receivable, Net, Current			53,500,000		53,500,000		53,300,000
Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax			(600,000)		(600,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Income Tax Expense (Benefit)			(1,000)	$ (4,000)	(2,000)	$ (5,000)
Interest Expense			(3,000)	$ (6,000)	(5,000)	$ (10,000)
Viridity Energy, Inc. [Member]
Business Combination, Consideration Transferred		$ 35,300,000
Business Combination, Contingent Consideration, Liability	$ 12,800,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 34,700,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	17 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Working Capital	$ 400,000
Goodwill	$ 13,900,000
Maximum [Member]
Cash, FDIC Insured Amount			$ 250,000		$ 250,000		$ 250,000
ONGP, LLC [Member] | Southern California Public Power Authority [Member]
Power Purchase Agreements, Number of Power Plants Portfolio			9		9
Expected Power Generating Capacity | MWh			150
Power Plant Usage Agreement Term			5 years
Power Purchase Agreements Term			26 years
Power Purchase Agreements, Fixed Price Per MWh | $ / MWh			75.5		75.5
Ormat Nevada ORTP LLC [Member] | Southern California Public Power Authority [Member] | Minimum [Member]
Expected Power Generating Capacity | MWh			135
Ormat Nevada ORTP LLC [Member] | Southern California Public Power Authority [Member] | Maximum [Member]
Expected Power Generating Capacity | MWh			185