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Investments (Tables)
 12 Months Ended
Dec. 31, 2017
Investments [Abstract]  
Unrealized Gain (Loss) on Investments [Table Text Block]
The following table summarizes unrealized gains and losses related to these investments as of December 31, 2017 and December 31, 2016:
Dollar amounts in millions
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Fair Value
December 31, 2017
$
0.4

 
$
5.6

 
$
—

 
$
6.0

 
 
 
 
 
 
 
 
December 31, 2016
$
0.4

 
$
4.4

 
$
—

 
$
4.8
Investments Classified by Contractual Maturity Date [Table Text Block]
debt securities classified as available for sale at December 31, 2017 exceed one year.