Investments (Tables)

3 Months Ended

Mar. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Components of Investments

Investments consisted of the following as of March 31, 2017:

			Unrealized		Unrealized			Fair
	Cost Basis		Gains		Losses			Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	10,597	$	—	$	—		$	10,597
Commercial paper		27,935		—		(26	)		27,909
U.S. Government agencies		15,305		—		(20	)		15,285
Corporate bonds and notes		236,525		4		(479	)		236,050
Subtotal		290,362		4		(525	)		289,841
Deferred compensation plan assets included in other assets		2,500		—		—			2,500
Total	$	292,862	$	4	$	(525	)	$	292,341

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

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:

	March 31, 2017				December 31, 2016
	Amortized		Fair		Amortized		Fair
	Costs		Value		Costs		Value
	(Thousands of dollars)
Due in one year or less	$	200,195	$	199,906	$	184,587	$	184,334
Due within two years		79,570		79,338		100,816		100,378
Total	$	279,765	$	279,244	$	285,403	$	284,712

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:

	March 31, 2017				December 31, 2016
	Average		Average		Average		Average
	Coupon		Days To		Coupon		Days To
	Rate (%)		Maturity		Rate (%)		Maturity
Commercial paper		1.035		140		1.001		184
U.S. Government agencies		1.056		355		0.970		400
Corporate bonds and notes		1.731		287		1.745		318

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:

	March 31, 2017					December 31, 2016
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Commercial paper	$	27,909	$	(26	)	$	27,867	$	(39	)
U.S. Government agencies		14,285		(20	)		13,263		(32	)
Corporate bonds and notes		207,512		(460	)		210,836		(602	)
Total	$	249,706	$	(506	)	$	251,966	$	(673	)

Investments in Continuous Unrealized Loss Position for More 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 more than twelve months as of:

	March 31, 2017					December 31, 2016
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Corporate bonds and notes	$	20,116	$	(19	)	$	24,196	$	(20	)
Total	$	20,116	$	(19	)	$	24,196	$	(20	)