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Note 1 - Business and Significant Accounting Policies (Details Textual) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 01, 2018
Mar. 15, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2018
Dividends, Common Stock, Cash                       $ 20,500,000 $ 25,700,000 $ 12,700,000  
Common Stock, Dividends, Per Share, Declared                       $ 0.41 $ 0.52 $ 0.26  
Accounts Receivable, Net, Current     $ 110,410,000       $ 80,807,000         $ 110,410,000 $ 80,807,000    
Provision for Doubtful Accounts                       0      
Interest Costs Capitalized                       7,224,000 3,287,000 $ 4,075,000  
Exploration Abandonment and Impairment Expense     1,796,000 303,000 $ 1,294,000 $ 863,000 $ 557,000   1,796,000 3,017,000 1,579,000  
Grants Received to Offset Exploration and Development Costs Incurred                       0 300,000 800,000  
Accumulated Amortization, Debt Issuance Costs     31,000,000       31,100,000         31,000,000 31,100,000    
Amortization of Debt Issuance Costs                       5,700,000 6,900,000 8,800,000  
Write off of Deferred Debt Issuance Cost                       600,000 100,000 500,000  
Deferred Costs, Leasing, Gross     4,200,000                 $ 4,200,000      
Lease Payment Term                       23 years      
Goodwill, Impairment Loss                       $ 0 0    
Impairment of Long-Lived Assets to be Disposed of                       0      
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax     1,400,000       1,200,000         1,400,000 1,200,000    
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax                       11,000,000 9,000,000 27,000,000  
Interest Expense     12,987,000 11,692,000 14,540,000 14,923,000 15,828,000 17,137,000 18,401,000 16,023,000   54,142,000 67,389,000 72,577,000  
Income Tax Expense (Benefit)     (29,669,000) $ 11,003,000 $ 6,369,000 $ 10,886,000 2,450,000 $ 11,988,000 $ 7,890,000 $ 9,509,000   $ (1,411,000) $ 31,837,000 $ (15,258,000)  
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                       35.00% 35.00% 35.00%  
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount                       42,896 102,793 467,766  
Research and Development Arrangement with Federal Government, Customer Funding to Offset Costs Incurred                       $ 0 $ 0 $ 0  
Retained Earnings [Member]                              
Common Stock, Dividends, Per Share, Declared                       $ 0.41 $ 0.52 $ 0.26  
Subsequent Event [Member]                              
Common Stock, Dividends, Per Share, Declared $ 0.23                            
Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]                              
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets                             $ 9.50
Subsequent Event [Member] | Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]                              
Cumulative Effect of New Accounting Principle in Period of Adoption                             24,100,000
Scenario, Forecast [Member]                              
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent                     21.00%        
Reclassification out of Accumulated Other Comprehensive Income [Member]                              
Interest Expense                       $ (16,000,000) $ (12,000,000) $ (44,000,000)  
Income Tax Expense (Benefit)                       $ (5,000,000) (3,000,000) (17,000,000)  
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     2,300,000       2,100,000         $ 2,300,000 2,100,000    
Amortization of Deferred Leasing Fees                       $ 200,000 $ 200,000 $ 200,000  
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Primary Customers [Member]                              
Concentration Risk, Percentage                       57.00% 60.00%    
Maximum [Member]                              
Cash, FDIC Insured Amount     250,000                 $ 250,000      
Finite-Lived Intangible Asset, Useful Life                       25 years      
Maximum [Member] | Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]                              
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets                             59.30
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] | Subsequent Event [Member] | Accounting Standards Update 2016-16 [Member]                              
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets                             $ 49.80
Minimum [Member] | Power Plants [Member]                              
Property, Plant and Equipment, Useful Life                       15 years      
UNITED STATES                              
Cash, Cash Equivalents, and Short-term Investments     21,200,000       72,500,000         $ 21,200,000 $ 72,500,000    
Foreign Countries [Member]                              
Cash, Cash Equivalents, and Short-term Investments     32,800,000       166,200,000         32,800,000 166,200,000    
Accounts Receivable, Net, Current     $ 78,100,000       $ 53,300,000         $ 78,100,000 $ 53,300,000