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Schedule II - Valuation and Qualifying Accounts (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Summary of Allowance for Doubtful Accounts and Revenue Adjustments [Table Text Block]
    2017   2016   2015
Balance at beginning of year   $
567
     $
499
     $
502
  
Provision for bad debt    
116
      
126
      
16
  
Write-offs, net of recoveries    
(31
 )    
(58
 )    
(19
 )
Balance at end of year   $
652
     $
567
     $
499