Note 1 - Business and Significant Accounting Policies (Details Textual) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Dividends, Common Stock, Cash									$ 25,700,000	$ 12,700,000	$ 9,600,000
Common Stock, Dividends, Per Share, Declared									$ 0.52	$ 0.26	$ 0.21
Cash, Cash Equivalents, and Short-term Investments	$ 230,214,000				$ 185,919,000				$ 230,214,000	$ 185,919,000	$ 40,230,000	$ 57,354,000
Accounts Receivable, Net, Current	80,807,000				55,301,000				80,807,000	55,301,000
Interest Costs Capitalized									3,287,000	4,075,000	3,206,000
Exploration Abandonment and Impairment Expense	303,000	$ 1,294,000	$ 863,000	$ 557,000	1,220,000	$ 185,000		$ 174,000	3,017,000	1,579,000	15,439,000
Grants Received to Offset Exploration and Development Costs Incurred									300,000	800,000	1,700,000
Accumulated Amortization, Debt Issuance Costs	31,100,000				37,200,000				31,100,000	37,200,000
Amortization of Debt Issuance Costs									6,900,000	8,800,000	6,500,000
Write off of Deferred Debt Issuance Cost									100,000	500,000	700,000
Deferred Costs, Leasing, Gross	4,200,000								$ 4,200,000
Lease Payment Term									23 years
Interest Expense	15,828,000	17,137,000	18,401,000	16,023,000	18,142,000	17,748,000	18,859,000	17,828,000	$ 67,389,000	72,577,000	84,654,000
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax									12,000	44,000	228,000
Income Tax Expense (Benefit)	2,450,000	$ 11,988,000	$ 7,890,000	$ 9,509,000	11,438,000	$ (38,211,000)	$ 6,056,000	$ 5,459,000	31,837,000	(15,258,000)	27,608,000
Research and Development Arrangement with Federal Government, Customer Funding to Offset Costs Incurred									$ 0	$ 0	$ 600,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									102,793	467,766	3,237,593
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Debt Issuance Costs	17,700,000				19,100,000				$ 17,700,000	$ 19,100,000
Provision for Doubtful Accounts									0
Line-of-credit Arrangements [Member]
Debt Issuance Costs, Line of Credit Arrangements, Net	1,100,000				1,000,000				1,100,000	1,000,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Interest Expense									(9,000)	(27,000)	$ (141,000)
Income Tax Expense (Benefit)									(3,000)	(17,000)	(87,000)
Puna Geothermal Ventures [Member]
Deferred Costs, Leasing, Gross	4,200,000								$ 4,200,000
Lease Payment Term									23 years
Deferred Costs, Leasing, Accumulated Amortization	2,100,000				2,000,000				$ 2,100,000	2,000,000
Amortization of Deferred Leasing Fees									$ 200,000	$ 200,000	200,000
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage									60.00%	66.00%
Maximum [Member]
Cash, FDIC Insured Amount	250,000								$ 250,000
Finite-Lived Intangible Asset, Useful Life									25 years
Maximum [Member] | Power Plants [Member]
Property, Plant and Equipment, Useful Life										30 years
Minimum [Member]
Finite-Lived Intangible Asset, Useful Life									13 years
Minimum [Member] | Power Plants [Member]
Property, Plant and Equipment, Useful Life									15 years
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments	72,500,000				19,000,000				$ 72,500,000	$ 19,000,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments	166,200,000				181,000,000				166,200,000	181,000,000
Accounts Receivable, Net, Current	$ 53,300,000				$ 27,800,000				53,300,000	27,800,000
Twilight Site in Oregon [Member]
Exploration Abandonment and Impairment Expense									$ 1,000,000
Wister Site in California [Member]
Exploration Abandonment and Impairment Expense											8,100,000
Mount Spur Site in Alaska [Member]
Exploration Abandonment and Impairment Expense											$ 7,300,000
Maui Site in Hawaii [Member]
Exploration Abandonment and Impairment Expense										$ 1,000,000