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Ownership percentage of common shares outstanding.

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Note 1 - General and Basis of Presentation 1 (Details Textual)						1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Aug. 14, 2019 USD ($)	Jul. 03, 2019 USD ($)	Jul. 02, 2019 USD ($)	May 30, 2019	Mar. 25, 2019 USD ($)	Oct. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2027	May 04, 2019 USD ($)	Apr. 29, 2019	Apr. 09, 2019 USD ($)	Jan. 04, 2019 USD ($)
Payments to Acquire Equity Method Investments										$ 3,096,000	$ 3,800,000
Proceeds from Issuance of Commercial Paper		$ 50,000,000.0								50,000,000	0
Exploration Abandonment and Impairment Expense							$ 0		$ 0	0	$ 119,000
Accounts Receivable, after Allowance for Credit Loss, Current, Total							139,226,000			139,226,000		$ 137,581,000
Product [Member]
Revenue, Remaining Performance Obligation, Amount							$ 167,400,000			$ 167,400,000
Revenue, Remaining Performance Obligation, Percentage							100.00%			100.00%
Kenya Power and Lighting Co LTD [Member]
Accounts Receivable, Past Due							$ 41,100,000			$ 41,100,000
Accounts Receivable, Past Due, Average Number of Days Overdue							60			60
Kenya Power and Lighting Co LTD [Member] \| Subsequent Event [Member]
Proceeds, Overdue Accounts Receivable						$ 10,800,000
ENNE [Member]
Accounts Receivable, Past Due							$ 20,100,000			$ 20,100,000
Proceeds, Overdue Accounts Receivable							$ 0
Accounts Receivable, Past Due, Average Number of Days Overdue							181			181
Accounts Receivable, Past Due, Average Number of Days Overdue, Number of Days Increase During Period							73
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Primary Customers [Member]
Concentration Risk, Percentage										59.00%		56.00%
Revenue Benchmark [Member] \| Customer Concentration Risk [Member] \| Sierra Pacific Power Company And Nevada Power Company [Member]
Concentration Risk, Percentage							15.10%		13.60%	16.70%	15.70%
Revenue Benchmark [Member] \| Customer Concentration Risk [Member] \| Southern California Public Power Authority [Member]
Concentration Risk, Percentage							16.40%		13.70%	17.70%	14.90%
Revenue Benchmark [Member] \| Customer Concentration Risk [Member] \| Kenya Power and Lighting Co LTD [Member]
Concentration Risk, Percentage							18.00%		18.60%	16.60%	16.70%
Revenue Benchmark [Member] \| Customer Concentration Risk [Member] \| Pacific Gas & Electric [Member]
Concentration Risk, Percentage							2.20%			1.40%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments, Total							$ 47,300,000			$ 47,300,000		$ 31,300,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments, Total							74,200,000			74,200,000		93,900,000
Accounts Receivable, after Allowance for Credit Loss, Current, Total							$ 109,600,000			109,600,000		$ 102,000,000.0
DEG 3 Loan Agreement [Member]
Debt Instrument, Interest Rate, Stated Percentage																	6.04%
Additional Migdal Loan [Member]
Debt Instrument, Face Amount					$ 50,000,000.0
Debt Instrument, Interest Rate, Stated Percentage					4.60%
Debt Instrument, Periodic Payment, Total					$ 2,100,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid					$ 18,500,000
Commercial Paper [Member] \| London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate		0.75%
Commercial Paper [Member] \| Minimum [Member]
Debt Instrument, Term		90 days
Commercial Paper [Member] \| Maximum [Member]
Debt Instrument, Term		5 years
Subsidiary of Medco Power [Member] \| Ijen Geothermal Project Company [Member]
Ownership Percentage Of Common Shares Outstanding			51.00%
Plumstriker and its Two Subsidiaries [Member] \| Plumstriker Loan Agreement [Member]
Debt Instrument, Face Amount														$ 23,500,000
Debt Instrument, Equal Quarterly Principal Installments, Percentage of Loan				1.25%
Plumstriker and its Two Subsidiaries [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Plumstriker Loan Agreement [Member]
Debt Instrument, Basis Spread on Variable Rate				3.50%
Guadeloupe 1 [Member] \| Loan Agreement with Société Général [Member]
Debt Instrument, Face Amount																$ 8,900,000
Debt Instrument, Interest Rate, Stated Percentage																1.52%
Guadeloupe 1 [Member] \| Loan Agreement with Bpifrance [Member]
Debt Instrument, Face Amount																$ 8,900,000
Debt Instrument, Interest Rate, Stated Percentage															1.93%
OrPower 4, Inc [Member] \| DEG 3 Loan Agreement [Member]
Debt Instrument, Face Amount																	$ 41,500,000
Ijen Geothermal Project Company [Member]
Equity Method Investment, Ownership Percentage			49.00%
Payments to Acquire Equity Method Investments			$ 2,700,000
McGinness Hills Phase III [Member] \| Forecast [Member]
Percentage of Distributable Cash Generated by Power Plant Received by Company													97.50%
Percentage of Tax Attributes Related to Power Plant Received by Company													95.00%
McGinness Hills Phase III [Member] \| Private Investor [Member]
Proceeds from Sale of Interest in Partnership Unit	$ 59,300,000
Additional Amount Receivable from Sale of Interest in Partnership Unit	9,000,000
Additional Amount Receivable from Sale of Interest in Partnership Unit, Upper Limit	$ 22,000,000
Puna Geothermal Power Plant [Member] \| Electricity Segment [Member]
Proceeds from Insurance Settlement, Operating Activities								$ 1,500,000
Puna Geothermal Power Plant [Member] \| Electricity Segment [Member] \| Cost of Sales [Member]
Proceeds from Insurance Settlement, Operating Activities										$ 9,300,000