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, 2019
	Amortized		Gross Unrealized				Estimated
	Cost		Gains		Losses		Fair Value
	(In millions)
Corporate debt securities	$	1,174	$	5	$	1	$	1,178
Mortgage-backed securities	420		1		1		420
Asset-backed securities	126		1		—		127
U.S. Treasury notes	86		—		—		86
Municipal securities	78		—		—		78
GSEs	49		—		—		49
Foreign securities	7		—		—		7
Certificates of deposit	1		—		—		1
Total	$	1,941	$	7	$	2	$	1,946



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



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	453	$	453
Due after one year through five years	957		962
Due after five years through ten years	171		171
Due after ten years	360		360
Total	$	1,941	$	1,946

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 amounted to $13 million in the year ended December 31, 2019. Gross realized investment losses were insignificant in the year ended December 31, 2019. Gross realized investment gains and losses for the years ended December 31, 2018 and 2017 were insignificant.

We have determined that unrealized losses at December 31, 2019 and 2018 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, 2019:



	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	$	222	$	1	167	$	—	$	—	—
Mortgage-backed securities	143		1		72	—		—		—
Total	$	365	$	2	239	$	—	$	—	—



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

Pursuant to the regulations governing our Health Plans segment subsidiaries, we maintain statutory deposits and deposits required by government authorities primarily in cash, cash equivalents, and U.S. Treasury securities. We also maintain restricted investments as protection against the insolvency of certain capitated providers. The use of these funds is limited as required by regulation in the various states in which we operate, or as needed in the event of insolvency of capitated providers. Therefore, such investments are reported as “Restricted investments” in the accompanying consolidated balance sheets.

We have the ability to hold these restricted investments until maturity, and as a result, we would not expect the value of these investments to decline significantly due to a sudden change in market interest rates. Our held-to-maturity restricted investments are carried at amortized cost, which approximates fair value, and mature in one year or less. The following table presents the balances of restricted investments:



	December 31,
	2019		2018
	(In millions)
Florida	$	12	$	32
New Mexico	21		43
Ohio	12		12
Puerto Rico	11		10
Other	23		23
Total Health Plans segment	$	79	$	120