Note 1 - Business and Significant Accounting Policies 1 (Details Textual) - USD ($)				3 Months Ended								12 Months Ended
	May 03, 2018	Jan. 01, 2018	Mar. 15, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends, Common Stock, Cash												$ 26,800,000		$ 25,700,000
Common Stock, Dividends, Per Share, Declared												$ 0.53	$ 0.41	$ 0.52
Cash, FDIC Insured Amount				$ 250,000								$ 250,000
Accounts Receivable, Net, Current, Total				137,581,000				$ 110,410,000				137,581,000	$ 110,410,000
Provision for Doubtful Accounts												0
Deferred Costs, Total								49,834,000					49,834,000
Interest Costs Capitalized												3,728,000	7,224,000	$ 3,287,000
Exploration Abandonment and Impairment Expense				3,000			$ 123,000	1,796,000				126,000	1,796,000	3,017,000
Grants Received to Offset Exploration and Development Costs Incurred												0	0	300,000
Accumulated Amortization, Debt Issuance Costs				21,800,000				31,000,000				21,800,000	31,000,000
Amortization of Debt Issuance Costs												4,600,000	5,700,000	6,900,000
Write off of Deferred Debt Issuance Cost												0	600,000	$ 100,000
Deferred Costs, Leasing, Gross								4,200,000					$ 4,200,000
Lease Payment Term													23 years
Impairment of Long-Lived Assets to be Disposed of												0
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax, Ending Balance				0				1,400,000				$ 0	$ 1,400,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent												21.00%	35.00%	35.00%
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount												176,400	42,900	102,800
Insured Event, Gain (Loss)												$ 4,463,000
Accounting Standards Update 2016-02 [Member]
Finance Lease, Right-of-Use Asset				27,000,000								27,000,000
Finance Lease, Liability, Total				30,000,000								30,000,000
Product Segment [Member]
Revenue, Remaining Performance Obligation, Amount				188,100,000								188,100,000
Galena 2 Power Purchase Agreement [Member]
Termination Fees				5,000,000				0				$ 5,000,000	0	0
Viridity Energy, Inc. [Member]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			19 years									19 years
Puna Geothermal Ventures [Member]
Deferred Costs, Leasing, Gross				4,200,000								$ 4,200,000
Lease Payment Term												23 years
Deferred Costs, Leasing, Accumulated Amortization				2,500,000				2,300,000				$ 2,500,000	$ 2,300,000
Amortization of Deferred Leasing Fees														$ 200,000
Puna Geothermal Power Plant [Member]
Insured Event, Gain (Loss)	$ 3,300,000											12,100,000
Accounting Standards Update 2016-16 [Member]
Deferred Costs, Total				49,800,000								49,800,000
Accounting Standards Update 2014-09 [Member]
Cumulative Effect on Retained Earnings, Net of Tax, Total		$ 24,000,000
Accounting Standards Update 2016-15 [Member] | Reclassifcation of Cash Paid From Investing Activities to Financing Activities [Member]
Prior Period Reclassification Adjustment												$ 8,000,000
Pacific Gas & Electric [Member]
Percentage of Total Revenue												1.90%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]
Concentration Risk, Percentage												56.00%	57.00%
UNITED STATES
Cash, Cash Equivalents, and Short-term Investments, Total				31,300,000				21,200,000				$ 31,300,000	$ 21,200,000
Foreign Countries [Member]
Cash, Cash Equivalents, and Short-term Investments, Total				93,900,000				32,800,000				93,900,000	32,800,000
Accounts Receivable, Net, Current, Total				$ 102,000,000				$ 78,100,000				$ 102,000,000	78,100,000
Maximum [Member]
Dividends, Common Stock, Cash													$ 20,500,000
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