Investments (Tables)

9 Months Ended

Sep. 30, 2016

Investments, Debt and Equity Securities [Abstract]

Components of Investments

Investments consisted of the following as of September 30, 2016:

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money market mutual funds	$	17,608	$	—	$	—		$	17,608
Commercial paper		32,820		—		(75	)		32,745
U.S. Government agencies		16,296		5		(3	)		16,298
Corporate bonds and notes		236,215		33		(355	)		235,893
Subtotal		302,939		38		(433	)		302,544
Deferred compensation plan assets included in other assets		2,392		—		—			2,392
Total	$	305,331	$	38	$	(433	)	$	304,936

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

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair 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:

	September 30, 2016				December 31, 2015
	Amortized		Fair		Amortized		Fair
	Costs		Value		Costs		Value
	(Thousands of dollars)
Due in one year or less	$	171,336	$	171,152	$	152,444	$	152,212
Due within two years		113,995		113,784		130,110		129,466
Total	$	285,331	$	284,936	$	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:    

	September 30, 2016				December 31, 2015
	Average		Average		Average		Average
	Coupon		Days To		Coupon		Days To
	Rate (%)		Maturity		Rate (%)		Maturity
Commercial paper		0.978		253		0.553		154
U.S. Government agencies		0.979		512		0.865		599
Corporate bonds and notes		1.643		330		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:

	September 30, 2016					December 31, 2015
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Commercial paper	$	31,753	$	(75	)	$	5,981	$	(5	)
U.S. Government agencies		7,001		(2	)		8,969		(30	)
Corporate bonds and notes		169,833		(320	)		232,347		(793	)
Total	$	208,587	$	(397	)	$	247,297	$	(828	)

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, 2016					December 31, 2015
			Unrealized					Unrealized
	Fair Value		Losses			Fair Value		Losses
	(Thousands of dollars)
Corporate bonds and notes	$	34,002	$	(36	)	$	28,866	$	(48	)
Total	$	34,002	$	(36	)	$	28,866	$	(48	)