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Fair Value Measurements (Notes)
12 Months Ended
Dec. 31, 2017
Fair Value Disclosures [Abstract]  
Fair Value Disclosures [Text Block] FAIR VALUE MEASUREMENTS

Available-for-sale investments included in cash equivalents at the end of each period were as follows (in thousands):
 
December 31, 2017
 
December 31, 2016
 
Adjusted Cost
 
Gross Unrealized Gains/Losses
 
Fair Value
 
Adjusted Cost
 
Gross Unrealized Gains/Losses
 
Fair Value
Municipal bonds and notes
$
13,320

 
$
(1
)
 
$
13,319

 
$
43,490

 
$

 
$
43,490

Corporate debt securities
49,432

 
(24
)
 
49,408

 
1,673

 
(2
)
 
1,671

Financial institution instruments
10,881

 

 
10,881

 
28,307

 

 
28,307

Total available-for-sale investments
$
73,633

 
$
(25
)
 
$
73,608

 
$
73,470

 
$
(2
)
 
$
73,468


 
Financial institution instruments include instruments issued or managed by financial institutions such as money market fund deposits and time deposits.