Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2017

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, 2017
	Total		(Level 1)		(Level 2)
	(Thousands of dollars)
Investments:
Money market mutual funds	$	5,278	$	5,278	$	—
Commercial paper	9,413		—		9,413
U.S. Government agencies	13,491		—		13,491
Corporate bonds and notes	188,235		—		188,235
	216,417		5,278		211,139
Deferred compensation plan assets	2,630		2,630		—
Total	$	219,047	$	7,908	$	211,139
			December 31, 2016
	Total		(Level 1)		(Level 2)
	(Thousands of dollars)
Investments:
Money market mutual funds	$	18,118	$	18,118	$	—
Commercial paper	27,867		—		27,867
U.S. government agencies	13,263		—		13,263
Corporate bonds and notes	243,582		—		243,582
	302,830		18,118		284,712
Deferred compensation plan assets	2,394		2,394		—
Total	$	305,224	$	20,512	$	284,712