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Marketable Securities Marketable Securities (Tables)
9 Months Ended
Sep. 30, 2018
Marketable Securities [Abstract]  
Available-for-sale Securities [Table Text Block]
The schedule of maturities at September 30, 2018 and December 31, 2017 is as follows:
 
Maturities as of
September 30, 2018
 
Maturities as of
December 31, 2017
 
1 Year or Less
 
1 to 4 Years
 
1 Year or Less
 
1 to 4 Years
Corporate bonds
$
26,451

 
$

 
$
11,979

 
$
29,523

Certificates of deposit
31,445

 

 
52,048

 

Total
$
57,896

 
$

 
$
64,027

 
$
29,523