Note 11 - Long-term Debt, Credit Agreements and Commercial Paper (Details Textual) $ in Thousands													1 Months Ended									12 Months Ended
	Jul. 03, 2019 USD ($)	May 30, 2019	Mar. 25, 2019 USD ($)	Mar. 22, 2018 USD ($)	Dec. 21, 2016 USD ($)	Nov. 29, 2016 USD ($)	Jul. 31, 2015 USD ($) MWh	Nov. 30, 2013 USD ($)	Sep. 26, 2013	Feb. 28, 2013 USD ($)	Nov. 30, 2012 USD ($)	Feb. 29, 2012	Oct. 31, 2019 USD ($)	Oct. 31, 2018 USD ($) MWh	May 31, 2016 USD ($)	Jun. 30, 2014 USD ($)	May 31, 2013 USD ($)	Nov. 30, 2012 USD ($)	Oct. 31, 2011 USD ($)	Aug. 31, 2011 USD ($)	Dec. 31, 2005 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 04, 2019 USD ($)	Apr. 29, 2019	Apr. 09, 2019 USD ($)	Apr. 04, 2019 USD ($)	Jan. 04, 2019 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 20, 2016 USD ($)	Sep. 30, 2016	Aug. 29, 2014 USD ($)	Aug. 23, 2012 USD ($)	Sep. 30, 2011 USD ($)
Letters of Credit Outstanding, Amount																							$ 213,800
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																							2.99
Proceeds from Issuance of Commercial Paper	$ 50,000																						$ 50,000	$ 0	$ 0
Commercial Paper																							50,000	0
Stockholders' Equity Attributable to Parent, Ending Balance																							$ 1,392,420	1,319,837
Percentage Of Company Assets																							46.60%
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance																							$ 1,515,410	1,445,096	$ 1,295,700							$ 1,168,272
Payments of Dividends, Total																							22,400
Commercial Paper [Member]
Commercial Paper																							$ 50,000
Covenant Requirement Minimum [Member]
Amount Available For Dividend Distribution Percent Of Cumulative Net Income																							35
Stockholders' Equity Attributable to Parent, Ending Balance																							$ 600,000
Percentage Of Company Assets																							25.00%
Minimum [Member] | Commercial Paper [Member]
Debt Instrument, Term	90 days
Maximum [Member] | Commercial Paper [Member]
Debt Instrument, Term	5 years
Maximum [Member] | Covenant Requirement Minimum [Member]
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																							6
London Interbank Offered Rate (LIBOR) [Member] | Commercial Paper [Member]
Debt Instrument, Basis Spread on Variable Rate	0.75%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member]
Debt Instrument, Term							12 years
Debt Instrument, Face Amount							$ 42,000
Power Utilization | MWh							20
Debt Instrument, Payment Frequency							144 months
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | Covenant Requirement Minimum [Member]
Debt Services, Coverage Ratio							1.15
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | Covenant Required to not Limit Dividends [Member]
Debt Services, Coverage Ratio							1.25
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | Minimum [Member]
Debt Instrument, Prepayment Premium Percentage							0.50%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | Maximum [Member]
Debt Instrument, Prepayment Premium Percentage							1.00%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Guaranteed [Member]
Debt Instrument, Basis Spread on Variable Rate							4.35%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Not Guaranteed [Member]
Debt Instrument, Basis Spread on Variable Rate							4.75%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited1 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate							1.25%
Loan Agreement with Banco Industrial S.A. and Westrust Bank (International) Limited [Member]
Debt Instrument, Face Amount																																				$ 310,000
Long-term Debt, Gross																							$ 26,300
Loan Agreement With OPIC [Member]
Debt Services, Coverage Ratio																							1.7
Long-term Debt, Gross																							$ 192,600
Debt Instrument, Prepayment Premium, First Two Years, Percentage																							2.00%
Debt Instrument, Prepayment Premium, After Year 3, Percentage																							1.00%
Restricted Cash and Cash Equivalents, Current, Total																							$ 2,500	$ 2,600
Letters of Credit Outstanding, Amount																							15,600
Loan Agreement With OPIC [Member] | Tranche One [Member]
Debt Instrument, Face Amount											$ 85,000							$ 85,000
Long-term Debt, Gross																							51,900
Prepayment Deposit On Debt																		20,500
Loan Agreement With OPIC [Member] | Tranche Two [Member]
Long-term Debt, Gross											135,000							135,000					$ 111,200
Proceeds from Issuance of Debt										$ 45,000
Debt Instrument, Interest Rate, Stated Percentage																							6.29%
Loan Agreement With OPIC [Member] | Tranche Three [Member]
Debt Instrument, Face Amount											45,000							45,000
Long-term Debt, Gross																							$ 29,600
Proceeds from Issuance of Debt								$ 45,000
Debt Instrument, Interest Rate, Stated Percentage																							6.12%
Loan Agreement With OPIC [Member] | Threshold For Loan Default [Member]
Debt Services, Coverage Ratio																							1.1
Loan Agreement With OPIC [Member] | Minimum [Member] | Historical [Member]
Debt Services, Coverage Ratio																							1.4
Olkaria III DEG [Member] | Tranche One [Member]
Debt Instrument, Face Amount											180,000							$ 180,000
Prepayment Penalty Charges											$ 1,500
Platanares Finance Agreement [Member] | OPIC [Member]
Long-term Debt, Gross																							$ 104,500
Prepayment Deposit On Debt																							$ 5,000
Proceeds from Issuance of Debt														$ 114,700
Debt Instrument, Interest Rate, Stated Percentage														7.02%
Debt Agreement, Maximum Borrowing Capacity														$ 124,700
Debt Instrument, Prepayment Premium Percentage, First or Second Year																							2.00%
Debt Instrument, Prepayment Premium Percentage, Third Year																							1.00%
Platanares Finance Agreement [Member] | Historical [Member] | OPIC [Member]
Debt Services, Coverage Ratio																							1.1
Platanares Finance Agreement [Member] | Debt Service Reserve [Member] | OPIC [Member]
Restricted Cash and Cash Equivalents, Current, Total																							$ 3,600
Letters of Credit Outstanding, Amount																							8,100
Platanares Finance Agreement [Member] | Well Drilling Reserve [Member] | OPIC [Member]
Restricted Cash and Cash Equivalents, Current, Total																							0
Letters of Credit Outstanding, Amount																							$ 2,000
Platanares Finance Agreement [Member] | Threshold For Loan Default [Member] | OPIC [Member]
Debt Services, Coverage Ratio																							1.2
Platanares Finance Agreement [Member] | Maximum [Member] | Forecast [Member] | OPIC [Member]
Proceeds from Issuance of Debt																						$ 10,000
Platanares [Member]
Current Power Generation | MWh														35
OrCal Senior Secured Notes [Member]
Debt Instrument, Face Amount																					$ 165,000
Proceeds from Issuance of Debt																					$ 161,100
Debt Instrument, Interest Rate, Stated Percentage																					6.21%
Debt Instrument, Issuance Costs																					$ 3,900
Repayments of Secured Debt													$ 15,000
Payment for Debt Extinguishment or Debt Prepayment Cost													$ 400
OFC Two Senior Secured Notes [Member]
Debt Instrument, Face Amount																																					$ 350,000
Government Guarantee Percent																							80.00%
Debtor-in-Possession Financing, Letters of Credit Outstanding																							$ 19,500
OFC Two Senior Secured Notes [Member] | Senior Notes [Member]
Secured Debt, Total																							203,000
OFC Two Senior Secured Notes [Member] | Wholly Owned Subsidiaries With Project Debt [Member]
Debt Instrument, Face Amount																																			$ 140,000
Debt Instrument, Interest Rate, Stated Percentage																																			4.61%
OFC Two Senior Secured Notes [Member] | Phase One Series A Senior Notes [Member]
Debt Instrument, Face Amount																			$ 151,700
Proceeds from Issuance of Debt																			$ 141,100
Debt Instrument, Interest Rate, Stated Percentage																			4.687%
Repayments of Long-term Debt, Total																$ 4,300
Other Reserves																							$ 0												$ 28,000
OFC Two Senior Secured Notes [Member] | Maximum [Member] | Historical [Member]
Debt Services, Coverage Ratio																							1.2
OFC Two Senior Secured Notes [Member] | Maximum [Member] | Projected [Member]
Debt Services, Coverage Ratio																							1.5
Don A. Cambell Senior Secured Notes [Member]
Debt Instrument, Face Amount						$ 92,500
Proceeds from Issuance of Debt						$ 87,100
Debt Instrument, Interest Rate, Stated Percentage						4.03%
Secured Debt, Total																							$ 78,200
Debt Instrument, Minimum Prepay Amount						$ 2,000
Don A. Cambell Senior Secured Notes [Member] | Minimum [Member]
Debt Services, Coverage Ratio						1.20
Prudential Capital Group Agreement [Member]
Debt Instrument, Term															7 years
Debt Instrument, Face Amount															$ 20,000
Long-term Debt, Gross																							18,300
Debt Instrument, Interest Rate, Stated Percentage															5.80%
Line of Credit Facility, Maximum Borrowing Capacity															$ 50,000
Debt Instrument, Remaining Balance															$ 16,000
DOE Loan Guarantee [Member]
Debt Instrument, Term																				21 years 6 months
Long-term Debt, Gross																							44,900
Debt Instrument, Interest Rate, Stated Percentage																				2.60%
Line of Credit Facility, Maximum Borrowing Capacity																				$ 96,800
Prudential Capital Purchase Agreement [member]
Debt Instrument, Term									24 years
Long-term Debt, Gross																							27,100
Debt Instrument, Interest Rate, Stated Percentage									6.75%
Senior Unsecured Bonds [Member]
Debt Instrument, Face Amount																							67,000
Proceeds from Issuance of Unsecured Debt																							250,000
Senior Unsecured Bonds [Member] | Series 3 Bonds [Member]
Debt Instrument, Face Amount																							137,000
Senior Unsecured Bonds, Series 2 [Member]
Debt Instrument, Interest Rate, Stated Percentage																																		3.70%
Senior Unsecured Bonds, Series 3 [Member]
Debt Instrument, Interest Rate, Stated Percentage																																		4.45%
Migdal Loan [Member]
Debt Instrument, Face Amount				$ 100,000
Debt Instrument, Interest Rate, Stated Percentage				4.80%
Debt Instrument, Periodic Payment, Total				$ 4,200
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid				$ 37,000
Debt Instrument Increase in Stated Interest Rate if Rating is Downgraded to ILA Negative				0.50%
Debt Instrument Increase in Stated Interest Rate Each Additional Downgrade				0.25%
Debt Instrument Decrease in Stated Interest Rate for Each Rating Upgrade				0.25%
Debt to EBITDA Ratio Threshold for Rate Increase				4.5
Debt Instrument Increase in Stated Interest Rate if Debt to EBITDA Ratio Exceeds Threshold				0.50%
Debt to EBITDA Ratio Requirement				6
Stockholders Equity, Debt Covenant, Minimum Threshold				$ 650,000
Stockholders Equity to Total Assets, Ratio				25.00%
Stockholders Equity, Debt Covenant, Minimum Threshold to Pay Dividends				$ 800,000
Dividends to Net Income, Ratio				50.00%
Migdal Loan [Member] | Minimum [Member]
Debt Instrument, Interest Rate, Stated Percentage				4.80%
Migdal Loan [Member] | Maximum [Member]
Debt Instrument, Increase in Stated Interest Rate				1.00%
Additional Migdal Loan [Member]
Debt Instrument, Face Amount			$ 50,000
Debt Instrument, Interest Rate, Stated Percentage			4.60%
Debt Instrument, Periodic Payment, Total			$ 2,100
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid			$ 18,500
DEG 2 Facility Agreement [Member]
Debt Instrument, Interest Rate, Stated Percentage					6.28%
Debt Instrument, Minimum Prepay Amount					$ 5,000
Line of Credit Facility, Maximum Borrowing Capacity																																	$ 50,000
Proceeds from Lines of Credit, Total					$ 50,000
Long-term Line of Credit, Total																							42,500
DEG 3 Loan Agreement [Member]
Long-term Debt, Gross																							37,100
Debt Instrument, Interest Rate, Stated Percentage																														6.04%
DEG 3 Loan Agreement [Member] | OrPower 4, Inc [Member]
Debt Instrument, Face Amount																														$ 41,500
Plumstriker Loan Agreement [Member] | Plumstriker and its Two Subsidiaries [Member]
Debt Instrument, Face Amount																										$ 23,500
Long-term Debt, Gross																							21,600
Debt Instrument, Equal Quarterly Principal Installments, Percentage of Loan		1.25%
Plumstriker Loan Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Plumstriker and its Two Subsidiaries [Member]
Debt Instrument, Basis Spread on Variable Rate		3.50%
Loan Agreement with Société Général [Member] | Guadeloupe 1 [Member]
Debt Instrument, Face Amount																												$ 8,900
Long-term Debt, Gross																							8,400
Debt Instrument, Interest Rate, Stated Percentage																												1.52%
Loan Agreement with Bpifrance [Member] | Guadeloupe 1 [Member]
Debt Instrument, Face Amount																													$ 8,900
Long-term Debt, Gross																							9,000
Debt Instrument, Interest Rate, Stated Percentage																											1.93%
Credit Agreements With Eight Commercial Banks [Member]
Letters of Credit Outstanding, Amount																							213,700
Line of Credit Facility, Maximum Borrowing Capacity																							505,000
Long-term Line of Credit, Total																							40,600
Credit Agreements With Eight Commercial Banks [Member] | Extensions of Credit in The Form of Loans and/or Letters of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity																							260,000
Credit Agreements With Eight Commercial Banks [Member] | Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity																							245,000
Credit Agreements With Eight Commercial Banks [Member] | Union Bank, N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity																							60,000
Credit Agreements With Eight Commercial Banks [Member] | HSBC Bank USA, N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity																							$ 35,000
Union Bank, N.A. [Member]
Debt Services, Coverage Ratio																							2.87
Letters of Credit Outstanding, Amount																							$ 59,500
Line of Credit Facility, Maximum Borrowing Capacity																							$ 60,000
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																							2.1
Amount Available For Dividend Distribution Percent Of Cumulative Net Income												2.0											1.0
Union Bank, N.A. [Member] | Minimum [Member]
Debt Services, Coverage Ratio												1.35
Union Bank, N.A. [Member] | Maximum [Member]
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio												4.5
HSBC Bank USA, N.A. [Member]
Debt Services, Coverage Ratio																							2.87
Letters of Credit Outstanding, Amount																							$ 25,500
Line of Credit Facility, Maximum Borrowing Capacity																							$ 35,000
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																							2.1
Amount Available For Dividend Distribution Percent Of Cumulative Net Income																							1.0
Line of Credit Facility, Borrowing Capacity Increase																	$ 10,000
HSBC Bank USA, N.A. [Member] | Minimum [Member]
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																	1.35
HSBC Bank USA, N.A. [Member] | Maximum [Member]
Debt To Earnings Before Interest Tax Depreciation And Amortization Ratio																	4.5
Amount Available For Dividend Distribution Percent Of Cumulative Net Income																	2.0
Surety Agreement [Member] | Chubb [Member]
Surety Bonds Available																															$ 200,000
Surety Bonds Outstanding																							$ 144,800