Investments (Tables)

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Components of Investments

Investments consisted of the following as of September 30, 2018:

	Cost Basis		Unrealized Gains		Unrealized Losses		Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	50,290	$	—	$	—	$	50,290
Deferred compensation plan assets included in other assets	852		—		—		852
Total	$	51,142	$	—	$	—	$	51,142

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
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

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:

	September 30, 2018				December 31, 2017
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	—	$	—	$	31,348	$	31,254
Due within two years	—		—		40,032		39,763
Total	$	—	$	—	$	71,380	$	71,017

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:

	September 30, 2018		December 31, 2017
	Average Coupon Rate (%)	Average Days To Maturity	Average Coupon Rate (%)	Average Days To Maturity
U.S. Government agencies	—	0	1.370	370
Corporate bonds and notes	—	0	1.766	392

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:

	September 30, 2018				December 31, 2017
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Thousands of dollars)
U.S. Government agencies	$	—	$	—	$	5,472	$	(28	)
Corporate bonds and notes	—		—		44,069		(271		)
Total	$	—	$	—	$	49,541	$	(299	)

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: 

	September 30, 2018				December 31, 2017
	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		)
Total	$	—	$	—	$	21,476	$	(65	)