Note 1 - General and Basis of Presentation (Details Textual) shares in Millions						1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 26, 2017 MWh	Jul. 26, 2017 shares	Jul. 10, 2017 USD ($)	Mar. 15, 2017 USD ($)	Dec. 29, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 MWh $ / MWh	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Repayments of Secured Debt										$ 14,270,000	$ 6,815,000
Repayments of Long-term Debt										55,226,000	40,997,000
Payments to Acquire Additional Interest in Subsidiaries										2,357,000
Goodwill						$ 20,667,000	$ 20,667,000			20,667,000		$ 6,650,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax							2,000		$ 2,000	5,000	7,000
Interest Expense							11,692,000		17,137,000	41,155,000	51,561,000
Income Tax Expense (Benefit)							11,003,000		11,988,000	28,258,000	29,387,000
Accumulated Other Comprehensive Income (Loss), Net of Tax						(5,634,000)	(5,634,000)			(5,634,000)		(7,732,000)
Exploration Abandonment and Impairment Expense							0		1,294,000	0	2,714,000
Cash, Cash Equivalents, and Short-term Investments						77,212,000	77,212,000		$ 90,066,000	77,212,000	$ 90,066,000	230,214,000	$ 185,919,000
Accounts Receivable, Net, Current						98,384,000	$ 98,384,000			98,384,000		$ 80,807,000
Provision for Doubtful Accounts										$ 0
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage										48.00%		60.00%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage							16.30%		14.40%	17.40%	18.60%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Kenya Power And Lighting Co LTD [Member]
Concentration Risk, Percentage							17.60%		15.10%	15.70%	16.40%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Southern California Public Power Authority [Member]
Concentration Risk, Percentage							9.10%		7.70%	8.90%	9.90%
Sales Revenue, Net [Member] | Customer Concentration Risk [Member] | Hyundai (Sarulla Goethermal Power Project) [Member]
Concentration Risk, Percentage							0.90%		24.00%	4.70%	14.00%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments						23,900,000	$ 23,900,000			$ 23,900,000		$ 72,500,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments						56,000,000	56,000,000			56,000,000		166,200,000
Accounts Receivable, Net, Current						67,500,000	67,500,000			67,500,000		53,300,000
Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax						(600,000)	(600,000)			(600,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest Expense							(6,000)		$ (3,000)	(9,000)	$ (11,000)
Income Tax Expense (Benefit)							(4,000)		$ (1,000)	(4,000)	$ (4,000)
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		$ 250,000
Ormat Nevada ORTP LLC [Member] | ORTP Transaction [Member]
Payments to Acquire Additional Interest in Subsidiaries			$ 2,400,000
Ormat Funding Corp [Member]
Repayments of Secured Debt										14,300,000
Gain (Loss) on Extinguishment of Debt						1,500,000
Amortization of Debt Issuance Costs						200,000
DEG [Member]
Gain (Loss) on Extinguishment of Debt						500,000
Amortization of Debt Issuance Costs										$ 400,000
Repayments of Long-term Debt						$ 11,800,000
ORIX Corporation [Member]
Number of Shares Purchased by Investor | shares		11
Percentage Ownership in Company Purchased by Investor		22.00%
Voting Rights, Number of Directors Designated		3
Number of Directors after Acquisition Transaction		9
Voting Rights, Effective Rate Cap		25.00%
Southern California Public Power Authority [Member] | Ormat Nevada ORTP LLC [Member] | Minimum [Member]
Expected Power Generating Capacity | MWh								135
Southern California Public Power Authority [Member] | Ormat Nevada ORTP LLC [Member] | Maximum [Member]
Expected Power Generating Capacity | MWh								185
Southern California Public Power Authority [Member] | ONGP, LLC [Member]
Expected Power Generating Capacity | MWh								150
Power Plant Usage Agreement Term								5 years
Power Purchase Agreements Term								26 years
Power Purchase Agreements, Number of Power Plants Portfolio								9
Power Purchase Agreements, Fixed Price Per MWh | $ / MWh								75.5
Geotermica Plantares [Member]
Expected Power Generating Capacity | MWh	35
Power Plant Usage Agreement Term	15 years
Geotermica Plantares [Member] | ENEE [Member]
Power Purchase Agreements Term	30 years