Fair Value (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Summary of Valuation of Investments and Financial Instruments Pricing Levels

The following tables summarize the valuation of our investments and financial instruments by the above pricing levels as of the valuation dates listed:

			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

			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

Summary of Combined Fair Value of Assets that Incurred Impairments

The below table summarizes the combined fair value of the assets that incurred impairments during the years ended December 31, 2016, 2015 and 2014, along with the amount of the impairment. The impairment charges were recorded in Impairment of assets. See Note 1 for additional information.

	Year Ended December 31,
	2016		2015		2014
	(Thousands of dollars)
Amount of impairment incurred	$	407	$	—	$	10,508
Combined fair value of assets incurring impairment, after impairment charges	$	1,380	$	—	$	6,000