Note 1 - General and Basis of Presentation 1 (Details Textual) $ in Thousands						1 Months Ended		2 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
		Sep. 02, 2021 USD ($)	Jul. 15, 2021 USD ($)	Jul. 13, 2021 USD ($)	Jul. 12, 2021 USD ($)	Oct. 31, 2021 USD ($)	Feb. 28, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2021 USD ($)
Loss from Catastrophes							$ 9,100
Exploration Abandonment and Impairment Expense									$ 0		$ 0	$ 0	$ 0
Cash, FDIC Insured Amount								$ 25,800	25,800			25,800		$ 18,900
Cash, Uninsured Amount								73,700	73,700			73,700		72,400
Accounts Receivable, after Allowance for Credit Loss, Current, Total								140,314	140,314			140,314		$ 149,170
Product [Member]
Revenue, Remaining Performance Obligation, Amount								66,600	66,600			66,600
Kenya Power and Lighting Co LTD [Member]
Accounts Receivable, Past Due								$ 33,800	$ 33,800		52,900	$ 33,800	52,900
Kenya Power and Lighting Co LTD [Member] | Minimum [Member]
Accounts Receivable, Past Due, Average Number of Days Overdue (Day)								73 days	73 days			73 days
Kenya Power and Lighting Co LTD [Member] | Maximum [Member]
Accounts Receivable, Past Due, Average Number of Days Overdue (Day)								83 days	83 days			83 days
Kenya Power and Lighting Co LTD [Member] | Subsequent Event [Member]
Proceeds from (Repayments of) Accounts Receivable Securitization, Total						$ 14,200
ENNE [Member]
Accounts Receivable, Past Due								$ 13,800	$ 13,800			$ 13,800			$ 7,400
ENNE [Member] | Subsequent Event [Member]
Proceeds from (Repayments of) Accounts Receivable Securitization, Total						$ 2,700
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage												52.00%		65.00%
Non-US [Member]
Accounts Receivable, after Allowance for Credit Loss, Current, Total								96,700	96,700			$ 96,700		$ 111,300
Hapoalim Loan Agreement [Member] | Bank Hapoalim B.M. [Member]
Debt Instrument, Face Amount					$ 125,000
Debt Instrument, Number of Semi-annual Payments					14
Debt Instrument, Periodic Payment, Total					$ 8,900
Debt Instrument, Term (Year)					7 years
Debt Instrument, Interest Rate, Stated Percentage					3.45%
Debt Instrument, Covenant, Maximum Debt to EBITDA Ratio					6
Debt Instrument, Covenant, Minimum Equity Capital, Amount					$ 750,000
Debt Instrument, Covenant, Equity Capital to Total Assets, Percent					25
HSBC Loan Agreement [Member] | HSBC Bank PLC [Member]
Debt Instrument, Face Amount			$ 50,000
Debt Instrument, Number of Semi-annual Payments			14
Debt Instrument, Periodic Payment, Total			$ 3,600
Debt Instrument, Term (Year)			7 years
Debt Instrument, Interest Rate, Stated Percentage			3.45%
Debt Instrument, Covenant, Maximum Debt to EBITDA Ratio			6
Debt Instrument, Covenant, Minimum Equity Capital, Amount			$ 750,000
Debt Instrument, Covenant, Equity Capital to Total Assets, Percent			25
Discount Loan Agreement [Member] | Israel Discount Bank Ltd. [Member]
Debt Instrument, Face Amount		$ 100,000
Debt Instrument, Number of Semi-annual Payments		16
Debt Instrument, Periodic Payment, Total		$ 6,250
Debt Instrument, Term (Year)		8 years
Debt Instrument, Interest Rate, Stated Percentage		2.90%
Debt Instrument, Covenant, Maximum Debt to EBITDA Ratio		6
Debt Instrument, Covenant, Minimum Equity Capital, Amount		$ 750,000
Debt Instrument, Covenant, Equity Capital to Total Assets, Percent		25
Electricity Segment [Member] | Puna Geothermal Power Plant [Member]
Business Interruption Insurance Recovery, Gross									15,800		20,400
General and Administrative Expense [Member]
Accounts Receivable, Credit Loss Expense (Reversal)										$ 3,000
Operating Expense [Member] | Electricity Segment [Member] | Puna Geothermal Power Plant [Member]
Gain on Business Interruption Insurance Recovery									$ 15,500		$ 17,800	15,500	$ 17,800
Two Contracted Geothermal Assets in Nevada [Member]
Payments to Acquire Businesses, Gross				$ 171,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Working Capital and Cash and Equivalents				$ 10,800
Business Acquisition, Percentage of Voting Interests Acquired				100.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finance Lease Liability	[1]			$ 258,400
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables	[2]			8,600
Two Contracted Geothermal Assets in Nevada [Member] | Electricity Segment [Member]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual								14,400
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual								4,300
Business Combination, Pro Forma Information, Tax and Finance Liability Interest Expense of Acquiree since Acquisition Date, Actual								$ 2,800
Two Contracted Geothermal Assets in Nevada [Member] | Power Purchase Agreement [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables				$ 7,800
Two Contracted Geothermal Assets in Nevada [Member] | General and Administrative Expense [Member]
Business Combination, Acquisition Related Costs												$ 4,700

[1] Finance liability is related to a sale and leaseback transaction entered into by the Seller in September 2015 under which it sold and leased back the undivided interests in the Dixie Valley power plant asset through June 2038. The lease transaction was accounted for by the Seller as a finance lease due to the Seller continued involvement and management of the power plant and the existence of an early buy-out option in September 2024. As per the accounting guidance, the Company retained the Seller's accounting of a "failed" sale and leaseback transaction and accordingly accounted for the liability as a financial liability. This financial liability, as well as the related power plant asset, were measured at their acquisition-date fair value.

[2] The gross amount of receivables due under the Dixie Valley and Beowawe PPAs is $7.8 million. These receivables were fully collected during the third quarter of 2021.