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Marketable Securities Marketable Securities (Tables)
6 Months Ended
Jun. 30, 2019
Marketable Securities [Abstract]  
Available-for-sale Securities [Table Text Block] The schedule of maturities at June 30, 2019 and December 31, 2018 is as follows:
 
Maturities as of
June 30, 2019
 
Maturities as of
December 31, 2018
 
1 Year or Less
 
1 to 4 Years
 
1 Year or Less
 
1 to 4 Years
Corporate bonds
$
2,150

 
$

 
$
24,617

 
$

Certificates of deposit
33,946

 

 
42,351

 

Total
$
36,096

 
$

 
$
66,968

 
$