Fair Value (Tables)

12 Months Ended

Dec. 31, 2017

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, 2017
	Total		(Level 1)		(Level 2)
			(Thousands of dollars)
Investments:
Money market mutual funds	$	5,601	$	5,601	$	—
Commercial paper	—		—		—
U.S. government agencies	7,467		—		7,467
Corporate bonds and notes	63,550		—		63,550
	76,618		5,601		71,017
Deferred compensation plan assets	2,685		2,685
Total	$	79,303	$	8,286	$	71,017

			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

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, 2017, 2016 and 2015, 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,
	2017		2016		2015
	(Thousands of dollars)
Amount of impairment incurred	$	368	$	407	$	—
Combined fair value of assets incurring impairment, after impairment charges	$	1,380	$	1,380	$	—