Investments (Tables)

3 Months Ended

Mar. 31, 2013

Investments [Abstract]

Components of investments

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
Investments:
Money Market Mutual Funds	$	24,392	$	—	$	—		$	24,392
Commercial Paper		7,496		—		(2	)		7,494
Corporate bonds and notes		28,796		30		(7	)		28,819
Subtotal		60,684		30		(9	)		60,705
Deferred compensation plan assets included in other assets		2,879		—		—			2,879
Total	$	63,563	$	30	$	(9	)	$	63,584

	Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
	(Thousands of dollars)
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 and fair value of debt investments based on maturities

	March 31, 2013				December 31, 2012
	Amortized Costs		Fair Value		Amortized Costs		Fair Value
	(Thousands of dollars)
Due in one year or less	$	34,673	$	34,697	$	35,480	$	35,470
Due within two years		1,619		1,616		—		—
Total	$	36,292	$	36,313	$	35,480	$	35,470

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

	March 31, 2013				December 31, 2012
	Average Coupon Rate (%)		Average Days To Maturity		Average Coupon Rate (%)		Average Days To Maturity
Commercial Paper		0.163		116		0.305		138
Corporate bonds and notes		3.041		108		2.965		112

Investments in continuous unrealized loss position for less than twelve months

	March 31, 2013					December 31, 2012
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Thousands of dollars)
Commercial Paper	$	7,494	$	(2	)	$	2,494	$	(2	)
Corporate bonds and notes		16,067		(7	)		16,771		(11	)
Total	$	23,561	$	(9	)	$	19,265	$	(13	)

Investments in continuous unrealized loss position for twelve months or more

	March 31, 2013				December 31, 2012
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Thousands of dollars)
Corporate bonds and notes	$	—	$	—	$	2,004	$	(.4	)
	$	—	$	—	$	2,004	$	(.4	)