Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2016

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:

			June 30, 2016
	Total		(Level 1)		(Level 2)
			(Thousands of dollars)
Investments:
Money market mutual funds	$	17,231	$	17,231	$	—
Commercial paper		12,393		—		12,393
U.S. Government agencies		28,312		—		28,312
Corporate bonds and notes		244,322		—		244,322
		302,258		17,231		285,027
Deferred compensation plan assets		2,386		2,386		—
Total	$	304,644	$	19,617	$	285,027

			December 31, 2015
	Total		(Level 1)		(Level 2)
Investments:
Money market mutual funds	$	18,181	$	18,181	$	—
Commercial paper		5,981		—		5,981
U.S. Government agencies		11,469		—		11,469
Corporate bonds and notes		264,228		—		264,228
		299,859		18,181		281,678
Deferred compensation plan assets		2,294		2,294		—
Total	$	302,153	$	20,475	$	281,678