Investments

We consider all of our investments, and our restricted investments that are classified as current assets, to be available-for-sale. As described further in Note 11, “Debt,” we maintain certain funds from the issuance of our 4.875% Notes in a segregated deposit account, a current asset reported as “Restricted investments” in the accompanying consolidated balance sheets. Such investments, while restricted as to their use and held in a segregated deposit account, are classified as available-for-sale based upon our contractual liquidity requirements.

The following tables summarize our investments as of the dates indicated:



	December 31, 2017
	Amortized		Gross Unrealized				Estimated
	Cost		Gains		Losses		Fair Value
	(In millions)
Corporate debt securities	$	1,591	$	1	$	4	$	1,588
U.S. Treasury notes	389		—		1		388
GSEs	255		—		2		253
Municipal securities	142		—		1		141
Asset-backed securities	117		—		—		117
Certificates of deposit	37		—		—		37
Subtotal - current investments	2,531		1		8		2,524
Corporate debt securities	101		—		—		101
U.S. treasury notes	68		—		—		68
Subtotal - restricted investments, current	169		—		—		169
	$	2,700	$	1	$	8	$	2,693



	December 31, 2016
	Amortized Cost		Gross Unrealized				Estimated Fair Value
	Amortized Cost		Gains		Losses		Estimated Fair Value
	(In millions)
Corporate debt securities	$	1,180	$	1	$	2	$	1,179
U.S. treasury notes	84		—		—		84
GSEs	232		—		1		231
Municipal securities	143		—		1		142
Asset-backed securities	69		—		—		69
Certificates of deposit	53		—		—		53
	$	1,761	$	1	$	4	$	1,758

There were no current restricted investments as of December 31, 2016.

The contractual maturities of our available-for-sale investments as of December 31, 2017 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,722	$	1,721
Due after one year through five years	972		966
Due after five years through ten years	6		6
	$	2,700	$	2,693

Gross realized gains and losses from sales of available-for-sale securities are calculated under the specific identification method and are included in investment income. Gross realized investment gains and losses for the years ended December 31, 2017, 2016 and 2015 were insignificant.

We have determined that unrealized losses at December 31, 2017 and 2016 are temporary in nature, because the change in market value for these securities resulted from fluctuating interest rates, rather than a deterioration of the creditworthiness of the issuers. So long as we maintain the intent and ability to hold these securities to maturity, we are unlikely to experience losses. In the event that we dispose of these securities before maturity, we expect that realized losses, if any, will be insignificant.

The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a continuous loss position for 12 months or more as of December 31, 2017.



	In a Continuous Loss Position for Less than 12 Months					In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value		Unrealized Losses		Total Number of Positions	Estimated Fair Value		Unrealized Losses		Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	1,297	$	3	561	$	94	$	1	69
U.S. Treasury notes	470		1		89	—		—		—
GSEs	173		1		69	95		1		47
Municipal securities	—		—		—	38		1		48
	$	1,940	$	5	719	$	227	$	3	164



	In a Continuous Loss Position for Less than 12 Months					In a Continuous Loss Position for 12 Months or More
	Estimated Fair Value		Unrealized Losses		Total Number of Positions	Estimated Fair Value		Unrealized Losses		Total Number of Positions
	(Dollars in millions)
Corporate debt securities	$	542	$	2	378	$	—	$	—	—
GSEs	198		1		73	—		—		—
Municipal securities	101		1		129	—		—		—
	$	841	$	4	580	$	—	$	—	—