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Note 5 - Investment In an Unconsolidated Company (Details Textual)
$ in Thousands
12 Months Ended
Jun. 04, 2014
USD ($)
Dec. 31, 2017
USD ($)
MWh
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
May 23, 2014
USD ($)
May 16, 2014
USD ($)
Oct. 31, 2013
USD ($)
Number of Commercial Lenders in Funding Consortium           6  
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax   $ 804 $ 1,185 $ 1,028      
Sarulla [Member] | Lenders Consortium [Member]              
Senior Notes, Total           $ 1,170,000  
Sarulla [Member]              
Jointly Owned Utility Plant, Proportionate Ownership Share   12.75%          
Expected Power Generating Capacity | MWh   330          
Power Plant Usage Agreement Term   30 years          
Number of Phases of Construction   3          
Power Utilization | MWh   110          
Percentage of Required Production Capacity   100.00%          
Percentage of Required Injection Capacity   85.00%          
Supply Commitment, Remaining Minimum Amount Committed             $ 255,600
Payments to Acquire Projects   $ 46,300          
Accumulated Cash Contributions to Acquire Projects   $ 58,200          
Contract Effective Date   April 4, 2013          
Sarulla [Member] | Interest Rate Swap [Member]              
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax   $ 800 1,200        
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax, Ending Balance   (5,100)          
Sarulla [Member] | Interest Rate Swap [Member] | Sarulla Project Company [Member]              
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax   $ 6,300 $ 9,300        
Sarulla [Member] | Lenders Consortium [Member] | Interest Rate Swap [Member]              
Proceeds from Issuance of Senior Long-term Debt $ 50,000            
Sarulla [Member] | Lenders Consortium [Member] | Interest Rate Swap [Member] | London Interbank Offered Rate (LIBOR) [Member]              
Debt Instrument, Basis Spread on Variable Rate 3.4565%            
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed Interest Rate [Member]              
Senior Notes, Total         $ 100,000    
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed LIBOR Interest Rate [Member]              
Senior Notes, Total         $ 1,070,000    
Sarulla [Member] | Lenders Consortium [Member] | Subject to Fixed LIBOR Interest Rate [Member] | Interest Rate Swap [Member]              
Senior Notes, Total $ 960,000