Note 1 - Business and Significant Accounting Policies 1 (Details Textual) $ / shares in Units, $ in Thousands				1 Months Ended	2 Months Ended	4 Months Ended	12 Months Ended
	Nov. 30, 2020 USD ($) $ / shares	Nov. 30, 2020 $ / shares shares	Nov. 18, 2020 $ / shares shares	Jan. 31, 2021 USD ($)	Feb. 28, 2021 USD ($)	Jul. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Jan. 01, 2020 USD ($)	Dec. 31, 2017 USD ($)
Cash, FDIC Insured Amount							$ 18,900	$ 12,900
Cash, Uninsured Amount							72,400	84,800
Accounts Receivable, after Allowance for Credit Loss, Current, Total							149,170	154,525
Interest Costs Capitalized							10,409	3,285	$ 3,728
Exploration Abandonment and Impairment Expense							0	0	126
Amortization of Debt Issuance Costs							3,500	5,400	4,600
Write off of Deferred Debt Issuance Cost							0	0	$ 0
Impairment of Long-Lived Assets to be Disposed of							0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax, Ending Balance							(900)	$ 1,500
Revenue, Remaining Performance Obligation, Amount							$ 33,400
Operating Lease, Weighted Average Discount Rate, Percent							5.00%
Finance Lease, Weighted Average Discount Rate, Percent							5.00%
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares							369,700	360,500	176,400
Payments of Dividends, Total							$ 22,500	$ 22,400	$ 26,800
Common Stock, Dividends, Per Share, Cash Paid (in dollars per share) | $ / shares							$ 0.44	$ 0.44	$ 0.53
Common Stock, Par or Stated Value Per Share (in dollars per share) | $ / shares			$ 0.001				$ 0.001	$ 0.001
Proceeds from Issuance of Common Stock							$ 339,471	$ 0	$ 0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance							1,941,437	1,515,410	1,445,096		$ 1,319,535
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance							550,103	487,873	422,222		$ 351,090
Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance								(755)	(58)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance								(755)	(58)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance										$ (800)
Electricity Segment [Member] | Puna Geothermal Power Plant [Member]
Business Interruption Insurance Recovery, Gross							28,600
Proceeds from Insurance Settlement, Operating Activities							4,700
Electricity Segment [Member] | Puna Geothermal Power Plant [Member] | Operating Expense [Member]
Gain on Business Interruption Insurance Recovery							20,700
Electricity Segment [Member] | Puna Geothermal Power Plant [Member] | Nonoperating Income (Expense) [Member]
Proceeds from Insurance Settlement, Operating Activities							600
COVID 19 [Member]
Proceeds from Issuance of Long-term Debt, Total						$ 400,000
Underwriting Agreement [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares			4,150,000
Shares Issued, Price Per Share (in dollars per share) | $ / shares			$ 74.00
Proceeds from Issuance of Common Stock	$ 339,500
Underwriting Agreement [Member] | Underwriters [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares		622,500
Shares Issued, Price Per Share (in dollars per share) | $ / shares	$ 74.00	$ 74.00
Galena 2 Power Purchase Agreement [Member]
Termination Fees							$ 0	$ 0	$ 5,000
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life (Year)							15 years
Standard Product Warranty, Term (Year)							1 year
Minimum [Member] | Viridity Energy, Inc. [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year)							6 years
Maximum [Member]
Finite-Lived Intangible Asset, Useful Life (Year)							29 years
Standard Product Warranty, Term (Year)							2 years
Maximum [Member] | Viridity Energy, Inc. [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life (Year)							19 years
Power Plants [Member] | Minimum [Member]
Property, Plant and Equipment, Useful Life (Year)							15 years
Power Plants [Member] | Maximum [Member]
Property, Plant and Equipment, Useful Life (Year)							30 years
Kenya Power and Lighting Co LTD [Member]
Accounts Receivable, Past Due							$ 48,900
Accounts Receivable, Past Due, Average Number of Days Overdue							78
Kenya Power and Lighting Co LTD [Member] | Subsequent Event [Member]
Proceeds, Overdue Accounts Receivable					$ 16,200
ENNE [Member]
Accounts Receivable, Past Due							$ 20,100
ENNE [Member] | Subsequent Event [Member]
Proceeds, Overdue Accounts Receivable				$ 2,900
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage							65.00%	58.00%
Non-US [Member]
Accounts Receivable, after Allowance for Credit Loss, Current, Total							$ 111,300	$ 118,800