Short -Term Investment (Tables)

12 Months Ended

Dec. 31, 2012

Short -Term Investments [Abstract]

Components of investments

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Short-term investments:
Money Market Mutual Funds	$	29,816	$	—	$	—		$	29,816
Commercial Paper		5,494		1		(2	)		5,493
Corporate bonds and notes		29,986		2		(11	)		29,977
Subtotal		65,296		3		(13	)		65,286
Deferred compensation plan assets included in other assets		2,687		—		—			2,687
Total	$	67,983	$	3	$	(13	)	$	67,973

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Short-term investments:
Money Market Mutual Funds	$	47,140	$	—	$	—		$	47,140
Commercial Paper		15,690		—		(12	)		15,678
Corporate bonds and notes		37,299		26		(116	)		37,209
Subtotal		100,129		26		(128	)		100,027
Deferred compensation plan assets included in other assets		2,807		—		—			2,807
Total	$	102,936	$	26	$	(128	)	$	102,834

Cost and fair value of debt investments based on maturities

	2012				2011
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	35,480	$	35,470	$	49,667	$	49,569
Due within two years		—		—		3,322		3,318
Total	$	35,480	$	35,470	$	52,989	$	52,887

Average coupon rate percentage and the average days to maturity of debt

	2012				2011
	Average		Average		Average		Average
	Coupon		Days To		Coupon		Days To
	Rate (%)		Maturity		Rate (%)		Maturity
Commercial Paper		0.305		138		0.191		116
Corporate bonds and notes		2.965		112		4.921		228

Investments in continuous unrealized loss position for less than twelve months

	2012					2011
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Commercial Paper	$	2,494	$	(2	)	$	15,678	$	(12	)
Corporate bonds and notes		16,771		(11	)		17,226		(48	)
Total	$	19,265	$	(13	)	$	32,904	$	(60	)

Investments in continuous unrealized loss position for twelve months or more

	2012					2011
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Corporate bonds and notes	$	2,004	$	(.4	)	$	5,172	$	(68	)
	$	2,004	$	(.4	)	$	5,172	$	(68	)