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Cash and Cash Equivalents and Investments in Marketable Securities (Tables)
12 Months Ended
Dec. 31, 2017
Cash, Cash Equivalents, and Short-term Investments [Abstract]  
Available-For-Sale Securities
The following is a summary of our marketable securities at December 31, 2016:
 
 
Amortized Cost
 
Gross Unrealized Gain
 
Gross Unrealized Loss
 
Fair Value
(Net Carrying Amount)
 
 
(In thousands)
December 31, 2016
 
 
 
 
 
 
 
 
Commercial paper
 
$
7,687

 
$
1

 
$
(1
)
 
$
7,687

Corporate debt securities
 
4,001

 

 

 
4,001

State and political subdivisions debt securities
 
412,462

 
1

 
(3
)
 
412,460

Total marketable securities
 
$
424,150

 
$
2

 
$
(4
)
 
$
424,148