Investments

Investments classified as available-for-sale, which consist primarily of debt securities, are stated at fair value. Unrealized gains and losses are excluded from earnings and reported as other comprehensive income, net of income tax effects. The cost of investments sold is determined in accordance with the specific identification method, and realized gains and losses are included in net investment income. We periodically assess our available-for-sale investments for other-than-temporary impairment. Any such other-than-temporary impairment loss is recognized as a realized loss, which is recorded through earnings, if related to credit losses.

We recognized an impairment related to certain investments in the energy and oil sector in the amount of $2.0 million during the year ended December 31, 2015. The amount represents the difference between the present value of the Company's best estimate of future cash flows using the latest performance indicators and the amortized cost basis. During the years ended December 31, 2014 and 2013, we recognized no losses from other-than-temporary impairments of our cash equivalents and available-for-sale investments.

We classified $27.6 million and $4.6 million as investments available-for-sale-noncurrent as of December 31, 2015 and 2014, respectively, because we did not intend to sell and we believed it may take longer than a year for such impaired securities to recover. This classification does not affect the marketability or the valuation of the investments, which are reflected at their market value as of December 31, 2015 and 2014.

As of December 31, 2015 and 2014, 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:



	2015
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Carrying Value
	(Dollars in millions)
Current:
Asset-backed securities	$	560.4	$	1.0	$	(5.8	)	$	555.6
U.S. government and agencies	32.0		—		(0.1		)	31.9
Obligations of states and other political subdivisions	913.6		24.3		(1.4		)	936.5
Corporate debt securities	703.7		1.0		(10.3		)	694.4
	$	2,209.7	$	26.3	$	(17.6	)	$	2,218.4
Noncurrent:
Corporate debt securities	34.2		—		(6.6		)	27.6
	$	34.2	$	—	$	(6.6	)	$	27.6



	2014
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Carrying Value
	(Dollars in millions)
Current:
Asset-backed securities	$	437.2	$	2.6	$	(1.9	)	$	437.9
U.S. government and agencies	36.5		—		—			36.5
Obligations of states and other political subdivisions	716.7		17.2		(1.7		)	732.2
Corporate debt securities	587.0		2.7		(5.3		)	584.4
	$	1,777.4	$	22.5	$	(8.9	)	$	1,791.0
Noncurrent:
Asset-backed securities	$	0.8	$	—	$	(0.2	)	$	0.6
Corporate debt securities	4.7		—		(0.7		)	4.0
	$	5.5	$	—	$	(0.9	)	$	4.6

As of December 31, 2015, the contractual maturities of our current investments available-for-sale and our investments available-for-sale-noncurrent were as follows:



	Amortized Cost		Estimated Fair Value
Current:	(Dollars in millions)
Due in one year or less	$	51.5	$	51.4
Due after one year through five years	514.2		511.7
Due after five years through ten years	579.9		582.1
Due after ten years	503.7		517.6
Asset-backed securities	560.4		555.6
Total current investments available-for-sale	$	2,209.7	$	2,218.4

	Amortized Cost		Estimated Fair Value
Noncurrent:	(Dollars in millions)
Due after one year through five years	$	15.8	$	13.1
Due after five years through ten years	17.2		13.4
Due after ten years	1.2		1.1
Total noncurrent investments available-for-sale	$	34.2	$	27.6

Proceeds from sales of investments available-for-sale during 2015 were $953.1 million. Gross realized gains and losses during 2015 totaled $6.7 million and $6.5 million, respectively. Proceeds from sales of investments available-for-sale during 2014 were $441.4 million. Gross realized gains and losses during 2014 totaled $5.7 million and $3.0 million, respectively. Proceeds from sales of investments available-for-sale during 2013 were $696.5 million. Gross realized gains and losses during 2013 totaled $26.4 million and $2.4 million, respectively.

The following tables show our investments’ fair values and gross unrealized losses for individual securities that have been in a continuous loss position through December 31, 2015 and December 31, 2014. These investments are interest-yielding debt securities of varying maturities. We have determined that the unrealized loss position for these securities is primarily due to market volatility. Generally, in a rising interest rate environment, the estimated fair value of fixed income securities would be expected to decrease; conversely, in a decreasing interest rate environment, the estimated fair value of fixed income securities would be expected to increase. These securities may also be negatively impacted by illiquidity in the market.

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, 2015:



	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	$	437.6	$	(5.2	)	$	34.0	$	(0.6	)	$	471.6	$	(5.8	)
U.S. government and agencies	28.2		(0.1		)	—		—			28.2		(0.1		)
Obligations of states and other political subdivisions	88.5		(0.7		)	19.7		(0.7		)	108.2		(1.4		)
Corporate debt securities	529.0		(9.4		)	27.3		(0.9		)	556.3		(10.3		)
	$	1,083.3	$	(15.4	)	$	81.0	$	(2.2	)	$	1,164.3	$	(17.6	)

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, 2015:



	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	$	18.0	$	(4.3	)	$	9.6	$	(2.3	)	$	27.6	$	(6.6	)
	$	18.0	$	(4.3	)	$	9.6	$	(2.3	)	$	27.6	$	(6.6	)

The following table shows the number of our individual securities-current that have been in a continuous loss position at December 31, 2015:



	Less than 12 Months	12 Months or More	Total
Asset-backed securities	266	39	305
U.S. government and agencies	8	—	8
Obligations of states and other political subdivisions	71	14	85
Corporate debt securities	505	32	537
	850	85	935

The following table shows the number of our individual securities-noncurrent that have been in a continuous loss position through December 31, 2015:



	Less than 12 Months	12 Months or More	Total
Corporate debt securities	21	9	30
	21	9	30

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, 2014:



	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	$	149.3	$	(0.5	)	$	112.5	$	(1.4	)	$	261.8	$	(1.9	)
U.S. government and agencies	20.7		—			—		—			20.7		—
Obligations of states and other political subdivisions	37.3		(0.1		)	104.8		(1.6		)	142.1		(1.7		)
Corporate debt securities	299.1		(3.9		)	56.0		(1.4		)	355.1		(5.3		)
	$	506.4	$	(4.5	)	$	273.3	$	(4.4	)	$	779.7	$	(8.9	)

The following table shows our noncurrent investments' fair value and gross unrealized losses for our individual securities that have been in a continuous loss position through December 31, 2014:



	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	$	—	$	—		$	0.6	$	(0.2	)	$	0.6	$	(0.2	)
Corporate debt securities	4.0		(0.7		)	—		—			4.0		(0.7		)
	$	4.0	$	(0.7	)	$	0.6	$	(0.2	)	$	4.6	$	(0.9	)