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Note 6 - Other Non-current Assets (Tables)
3 Months Ended
Mar. 31, 2021
Notes Tables  
Schedule of Other Assets, Noncurrent [Table Text Block]
  

March 31,

  

December 31,

 
  

2021

  

2020

 
Permitting costs(1) $7,385  $7,385 

Enterprise resource planning system, net

  1,451   1,805 
Rio Grande Site Lease initial direct costs  8,642   7,109 

Total other non-current assets, net

 $17,478  $16,299