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



	March 31, 2020
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		Value
	(In millions)
Corporate debt securities	$	1,277	$	4	$	21	$	1,260
Mortgage-backed securities	453		6		8		451
Asset-backed securities	144		—		1		143
Municipal securities	138		—		—		138
Certificates of deposit	7		—		—		7
GSEs	6		—		—		6
U.S. Treasury notes	5		—		—		5
Totals	$	2,030	$	10	$	30	$	2,010



	December 31, 2019
	Amortized		Gross Unrealized				Estimated Fair
	Cost		Gains		Losses		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
Municipal securities	78		—		—		78
Certificates of deposit	1		—		—		1
GSEs	49		—		—		49
U.S. Treasury notes	86		—		—		86
Foreign securities	7		—		—		7
Totals	$	1,941	$	7	$	2	$	1,946

The contractual maturities of our available-for-sale investments as of March 31, 2020 are summarized below:



	Amortized Cost		Estimated Fair Value
	(In millions)
Due in one year or less	$	375	$	375
Due after one year through five years	1,032		1,017
Due after five years through ten years	183		181
Due after ten years	440		437
Totals	$	2,030	$	2,010

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 $5 million in the three months ended March 31, 2020. Gross realized investment losses were insignificant in the three months ended March 31, 2020. Gross realized investment gains and losses were insignificant in the three months ended March 31, 2019.

We have determined that unrealized losses at March 31, 2020, and December 31, 2019, have primarily resulted from fluctuating interest rates, rather than a deterioration of the creditworthiness of the issuers. Therefore, we have determined that an allowance for credit losses is not necessary. 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 March 31, 2020:



	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	$	805	$	21	627	$	—	$	—	—
Mortgage-backed securities	211		8		128	—		—		—
Asset-backed securities	90		1		70	—		—		—
Totals	$	1,106	$	30	825	$	—	$	—	—



	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	—		—		—
Totals	$	365	$	2	239	$	—	$	—	—

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 regulations 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.