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Note 9 - Intangible Assets and Goodwill (Details Textual) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 15, 2017
Finite-Lived Intangible Assets, Net $ 85,420,000      
Amortization of Intangible Assets 6,900,000 $ 4,400,000 $ 3,300,000  
Increase (Decrease) in Intangible Assets, Current 35,600,000 33,000,000 500,000  
Finite Lived Intangible Assets, Disposals 0 0 $ 0  
Goodwill 21,037,000 6,650,000    
Viridity Energy, Inc. [Member]        
Goodwill       $ 13,400,000
Goodwill, Acquired During Period 13,500,000      
Guadeloupe Power Plant Transaction [Member]        
Goodwill, Acquired During Period   7,100,000    
Power Purchase Agreements and Intangible Assets Related to Storage Activities [Member]        
Finite-Lived Intangible Assets, Net 85,400,000 52,800,000    
Finite-Lived Intangible Assets, Accumulated Amortization 50,000,000 $ 42,800,000    
Intangible Assets Related to Storage Activities [Member]        
Finite-Lived Intangible Assets, Net 33,800,000      
Finite-Lived Intangible Assets, Accumulated Amortization $ 1,700,000