Fair Value (Tables)

12 Months Ended

Dec. 31, 2014

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, 2014
	Total		(Level 1)		(Level 2)
Investments:
Money Market Mutual Funds	$	68,612	$	68,612	$	—
Municipal bonds and notes		1,502		—		1,502
Corporate bonds and notes		130,615		—		130,615
		200,729		68,612		132,117
Deferred compensation plan assets		2,386		2,386		—
Total	$	203,115	$	70,998	$	132,117

			December 31, 2013
	Total		(Level 1)		(Level 2)
Investments:
Money Market Mutual Funds	$	18,470	$	18,470	$	—
Commercial Paper		2,999		—		2,999
Municipal bonds and notes		1,499		—		1,499
Corporate bonds and notes		77,633		—		77,633
		100,601		18,470		82,131
Deferred compensation plan assets		2,109		2,109		—
Total	$	102,710	$	20,579	$	82,131

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, 2014 and 2013, along with the amount of the impairment. The impairment charges were recorded in Impairment of assets.

	Year Ended December 31,
	2014		2013
	(Thousands of dollars)
Amount of impairment incurred	$	10,508	$	1,648
Combined fair value of assets incurring impairment	$	6,000	$	2,400