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Schedule II - Valuation and Qualifying Accounts (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Schedule II Valuation and Qualifying Accounts [Table Text Block]
   
Balance at beginning of period
    
Additions
    
Deduction
    
Balance at end of period
  
Year ended December 31, 2018
                                
Allowance for doubtful accounts
   $
—
     $
—
     $
—
     $
—
  
                                 
Year ended December 31, 2017
                                
Allowance for doubtful accounts
   $
—
     $
—
     $
—
     $
—
  
                                 
Year ended December 31, 2016
                                
Allowance for doubtful accounts
   $
25
     $
—
     $
25
     $
—