Investments (Tables)

12 Months Ended

Dec. 31, 2012

Dec. 31, 2011

Investments [Abstract]

Fair Value of Available-for-Sale Investments

As of December 31, 2012 and 2011, the amortized cost, gross unrealized holding gains and losses, and fair value of our current investments available-for-sale and our investments available-for-sale-noncurrent, after giving effect to other-than-temporary impairments were as follows:

	2012
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Carrying Value
	(Dollars in millions)
Current:
Asset-backed securities	$	499.7	$	19.6	$	(0.2	)	$	519.1
U.S. government and agencies	25.9		—		—			25.9
Obligations of states and other political subdivisions	819.9		24.2		(2.0		)	842.1
Corporate debt securities	408.4		17.5		(0.5		)	425.4
	$	1,753.9	$	61.3	$	(2.7	)	$	1,812.5

	2011
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Carrying Value
	(Dollars in millions)
Current:
Asset-backed securities	$	611.9	$	10.6	$	(0.2	)	$	622.3
U.S. government and agencies	32.5		—		—			32.5
Obligations of states and other political subdivisions	498.7		19.5		(0.1		)	518.1
Corporate debt securities	385.0		4.3		(4.2		)	385.1
	$	1,528.1	$	34.4	$	(4.5	)	$	1,558.0
Noncurrent:
Corporate debt securities	$	2.4	$	—	$	(0.3	)	$	2.1

Available for Sale Current and Noncurrent Investments Classified by Contractual Maturity Date

As of December 31, 2012, the contractual maturities of our current investments available-for-sale were as follows:

	Amortized Cost		Estimated Fair Value
Current:	(Dollars in millions)
Due in one year or less	$	34.2	$	34.3
Due after one year through five years	211.6		218.7
Due after five years through ten years	485.3		504.5
Due after ten years	523.1		535.9
Asset-backed securities	499.7		519.1
Total current investments available-for-sale	$	1,753.9	$	1,812.5

Schedule of Unrealized Loss on Investments

The following table shows our current investments' fair values and gross unrealized losses for individual securities in a continuous loss position as of December 31, 2012:

	Less than 12 Months					12 Months or More				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Dollars in millions)
Asset-backed securities	$	54.9	$	(0.2	)	$	0.1	$	—	$	55.0	$	(0.2	)
U.S. government and agencies	10.1		—			—		—		10.1		—
Obligations of states and other political subdivisions	192.1		(2.0		)	0.2		—		192.3		(2.0		)
Corporate debt securities	45.9		(0.5		)	—		—		45.9		(0.5		)
	$	303.0	$	(2.7	)	$	0.3	$	—	$	303.3	$	(2.7	)

The following table shows our current investments’ fair values and gross unrealized losses for individual securities that have been in a continuous loss position through December 31, 2011:

	Less than 12 Months					12 Months or More					Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
	(Dollars in millions)
Asset-backed securities	$	30.5	$	(0.2	)	$	1.1	$	—		$	31.6	$	(0.2	)
U.S. government and agencies	—		—			—		—			—		—
Obligations of states and other political subdivisions	7.5		—			3.0		(0.1		)	10.5		(0.1		)
Corporate debt securities	149.3		(4.1		)	1.4		(0.1		)	150.7		(4.2		)
	$	187.3	$	(4.3	)	$	5.5	$	(0.2	)	$	192.8	$	(4.5	)

The following table shows our noncurrent investments’ fair values and gross unrealized losses for individual securities that have been in a continuous loss position through December 31, 2011:

	Less than 12 Months					12 Months or More				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(Dollars in millions)
Corporate debt securities	$	2.1	$	(0.3	)	$	—	$	—	$	2.1	$	(0.3	)

Number of Securities Included in Loss Position of Current and Noncurrent Investments

The following table shows the number of our individual securities-current that have been in a continuous loss position at December 31, 2012:

	Less than 12 Months	12 Months or More	Total
Asset-backed securities	25	3	28
U.S. government and agencies	1	—	1
Obligations of states and other political subdivisions	82	1	83
Corporate debt securities	50	—	50
	158	4	162