Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]  
Schedule of Marketable Securities [Table Text Block]
 
March 31, 2018
 
December 31, 2017
 
Cost
 
Unrealized
Gain (Loss)
 
Fair
Value
 
Cost
 
Unrealized
Gain (Loss)
 
Fair
Value
U.S. government securities
$
3

 
$
—

 
$
3

 
$
3

 
$
—

 
$
3

Corporate securities
5

 


 
5

 
5

 


 
5

Certificates of deposit
28

 


 
28

 
27

 


 
27

Other
5

 


 
5

 
4

 
1

 
5

Total marketable securities
$
41

 
$
—

 
$
41

 
$
39

 
$
1

 
$
40