Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Components of Investments

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

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	5,601	$	—	$	—		$	5,601
Commercial paper	—		—		—			—
U.S. government agencies	7,501		—		(34		)	7,467
Corporate bonds and notes	63,880		—		(330		)	63,550
Subtotal	76,982		—		(364		)	76,618
Deferred compensation plan assets included in other assets	2,685		—		—			2,685
Total	$	79,667	$	—	$	(364	)	$	79,303

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

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair 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 December 31,

	2017				2016
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	31,348	$	31,254	$	184,587	$	184,334
Due within two years	40,032		39,763		100,816		100,378
Total	$	71,380	$	71,017	$	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 December 31,

	2017		2016
	Average Coupon Rate (%)	Average Days To Maturity	Average Coupon Rate (%)	Average Days To Maturity
Commercial paper	—	0	1.001	184
U.S. government agencies	1.370	370	0.970	400
Corporate bonds and notes	1.766	392	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 December 31,

	2017					2016
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Commercial paper	$	—	$	—		$	27,867	$	(39	)
U.S. government agencies	5,472		(28		)	13,263		(32		)
Corporate bonds and notes	44,069		(271		)	210,836		(602		)
Total	$	49,541	$	(299	)	$	251,966	$	(673	)

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,

	2017					2016
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
U.S. government agencies	$	1,994	$	(6	)	$	—	$	—
Corporate bonds and notes	$	19,482	$	(59	)	$	24,196	$	(20	)
Total	$	21,476	$	(65	)	$	24,196	$	(20	)