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Series B Debentures, Net of Current Maturities (Tables)
12 Months Ended
Dec. 31, 2019
Debt Disclosure [Abstract]  
Schedule of Long-term Debt Instruments [Table Text Block]
   December 31, 
   2018   2019 
         
Series B Debentures  $79,185   $69,287 
Less: Current maturities   (9,898)   (9,898)
Less: Unamortized debt discounts and issuance costs   (710)   (539)
           
   $68,577   $58,850