Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 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:

			March 31, 2017
	Total		(Level 1)		(Level 2)
			(Thousands of dollars)
Investments:
Money market mutual funds	$	10,597	$	10,597	$	—
Commercial paper		27,909		—		27,909
U.S. Government agencies		15,285		—		15,285
Corporate bonds and notes		236,050		—		236,050
		289,841		10,597		279,244
Deferred compensation plan assets		2,500		2,500		—
Total	$	292,341	$	13,097	$	279,244
			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