Investments

We consider all of our investments classified as current assets to be available-for-sale. The following tables summarize our current investments as of the dates indicated:



	December 31, 2018
	Amortized		Gross Unrealized				Estimated
	Cost		Gains		Losses		Fair Value
	(In millions)
Corporate debt securities	$	1,131	$	—	$	8	$	1,123
U.S. Treasury notes	181		—		—		181
GSEs	164		—		1		163
Municipal securities	115		—		1		114
Asset-backed securities	83		—		1		82
Certificates of deposit	14		—		—		14
Other	4		—		—		4
Total current investments	$	1,692	$	—	$	11	$	1,681



	December 31, 2017
	Amortized Cost		Gross Unrealized				Estimated Fair Value
	Amortized Cost		Gains		Losses		Estimated 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 - current restricted investments	169		—		—		169
	$	2,700	$	1	$	8	$	2,693

The contractual maturities of our current investments as of December 31, 2018 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	1,000	$	997
Due after one year through five years	692		684
	$	1,692	$	1,681

As discussed further in Note 11, “Debt,” the 4.875% Notes’ indenture required us to hold a portion of the net proceeds from their issuance in a segregated account to be used to settle the conversion of the 1.625% Convertible Notes. Prior to September 30, 2018, this account was reported as a current asset, entitled “Restricted investments,” in the accompanying consolidated balance sheets. Because this account was used to settle the conversion of the 1.625% Convertible Notes in the third quarter of 2018, current restricted investments, as of December 31, 2018, was reduced to zero.

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, 2018, 2017 and 2016 were insignificant.

We have determined that unrealized losses at December 31, 2018 and 2017 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, 2018.



	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	$	509	$	3	285	$	412	$	5	298
GSEs	—		—		—	127		1		76
Municipal securities	—		—		—	87		1		90
Asset backed securities	—		—		—	68		1		52
	$	509	$	3	285	$	694	$	8	516



	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