Fair Value Measurements (Tables)

9 Months Ended

Sep. 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:

			September 30, 2016
	Total		(Level 1)		(Level 2)
			(Thousands of dollars)
Investments:
Money market mutual funds	$	17,608	$	17,608	$	—
Commercial paper		32,745		—		32,745
U.S. Government agencies		16,298		—		16,298
Corporate bonds and notes		235,893		—		235,893
		302,544		17,608		284,936
Deferred compensation plan assets		2,392		2,392		—
Total	$	304,936	$	20,000	$	284,936

			December 31, 2015
	Total		(Level 1)		(Level 2)
			(Thousands of dollars)
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