Investments (Tables)

12 Months Ended

Dec. 31, 2016

Investments, Debt and Equity Securities [Abstract]

Components of Investments

Investments consisted of the following as of December 31, 2016:

			Unrealized		Unrealized			Fair
	Cost Basis		Gains		Losses			Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	18,118	$	—	$	—		$	18,118
Commercial paper		27,906		—		(39	)		27,867
U.S. government agencies		13,295		—		(32	)		13,263
Corporate bonds and notes		244,202		2		(622	)		243,582
Subtotal		303,521		2		(693	)		302,830
Deferred compensation plan assets included in other assets		2,394		—		—			2,394
Total	$	305,915	$	2	$	(693	)	$	305,224

Investments consisted of the following as of December 31, 2015:

			Unrealized		Unrealized			Fair
	Cost Basis		Gains		Losses			Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	18,181	$	—	$	—		$	18,181
Commercial paper		5,986		—		(5	)		5,981
U.S. government agencies		11,499		—		(30	)		11,469
Corporate bonds and notes		265,069		—		(841	)		264,228
Subtotal		300,735		—		(876	)		299,859
Deferred compensation plan assets included in other assets		2,294		—		—			2,294
Total	$	303,029	$	—	$	(876	)	$	302,153

Cost and Fair Value of Debt Investments Based on Maturities

The following table presents the cost and fair value of our debt investments based on maturities as of December 31,

	2016				2015
	Amortized		Fair		Amortized		Fair
	Costs		Value		Costs		Value
	(Thousands of dollars)
Due in one year or less	$	184,587	$	184,334	$	152,444	$	152,212
Due within two years		100,816		100,378		130,110		129,466
Total	$	285,403	$	284,712	$	282,554	$	281,678

Average Coupon Rate Percentage and Average Days to Maturity of Debt

The following table presents the average coupon rate percentage and the average days to maturity of our debt investments as of December 31,

	2016				2015
	Average		Average		Average		Average
	Coupon		Days To		Coupon		Days To
	Rate (%)		Maturity		Rate (%)		Maturity
Commercial paper		1.001		184		0.553		154
U.S. government agencies		0.970		400		0.865		599
Corporate bonds and notes		1.745		318		1.757		331

Investments in Continuous Unrealized Loss Position for Less Than Twelve Months

The following table presents the fair value and unrealized losses related to our investments that have been in a continuous unrealized loss position for less than twelve months as of December 31,

	2016					2015
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Commercial paper	$	27,867	$	(39	)	$	5,981	$	(5	)
U.S. government agencies		13,263		(32	)		8,969		(30	)
Corporate bonds and notes		210,836		(602	)		232,347		(793	)
Total	$	251,966	$	(673	)	$	247,297	$	(828	)

Investments in Continuous Unrealized Loss Position For Twelve Months Or More

The following table presents the fair value and unrealized losses related to our investments that have been in a continuous unrealized loss position for twelve months or more as of December 31,

	2016					2015
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Corporate bonds and notes	$	24,196	$	(20	)	$	28,866	$	(48	)
Total	$	24,196	$	(20	)	$	28,866	$	(48	)