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Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2018
Fair Value Disclosures [Abstract]  
Summary of Valuation of Investments and Financial Instruments Pricing Levels
The following table summarizes the valuation of our investments and financial instruments by the above pricing levels as of the valuation dates listed:
 
 
September 30, 2018
 
 
Total
 
(Level 1)
 
(Level 2)
 
 
(Thousands of dollars)
Investments:
 
 
 
 
 
 
Money market mutual funds
 
$
50,290

 
$
50,290

 
$

Deferred compensation plan assets
 
852

 
852

 

Total
 
$
51,142

 
$
51,142

 
$

 
 
 
 
 
 
 
 
 
December 31, 2017
 
 
Total
 
(Level 1)
 
(Level 2)
 
 
(Thousands of dollars)
Investments:
 
 
 
 
Money market mutual funds
 
$
5,601

 
$
5,601

 
$

U.S. government agencies
 
7,467

 

 
7,467

Corporate bonds and notes
 
63,550

 

 
63,550

 
 
76,618

 
5,601

 
71,017

Deferred compensation plan assets
 
2,685

 
2,685

 

Total
 
$
79,303

 
$
8,286

 
$
71,017