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:


	March 31, 2024
	Amortized Cost		Gross Unrealized				Estimated Fair Value
			Gains		Losses

	(In millions)
Corporate debt securities	$	2,908	$	8	$	63	$	2,853
Mortgage-backed securities	963		4		44		923
Asset-backed securities	390		1		10		381
Municipal securities	183		—		7		176
U.S. Treasury notes	44		—		—		44



Other	49		—		2		47
Total	$	4,537	$	13	$	126	$	4,424


	December 31, 2023
	Amortized Cost		Gross Unrealized				Estimated Fair Value
			Gains		Losses

	(In millions)
Corporate debt securities	$	2,781	$	16	$	65	$	2,732
Mortgage-backed securities	951		4		44		911
Asset-backed securities	376		1		12		365
Municipal securities	172		—		6		166
U.S. Treasury notes	40		—		—		40


Other	47		—		2		45
Total	$	4,367	$	21	$	129	$	4,259


	Amortized Cost		Estimated Fair Value

	(In millions)
Due in one year or less	$	634	$	625
Due after one year through five years	2,419		2,366
Due after five years through ten years	431		427
Due after ten years	1,053		1,006
Total	$	4,537	$	4,424

In the three months ended March 31, 2024, and 2023, maturities and redemptions of available-for-sale securities amounted to $188 million and $118 million, respectively, and sales amounted to $23 million and $253 million, respectively. 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 were insignificant for the three months ended March 31, 2024, and 2023. Gross realized investment losses amounted to $2 million and $10 million in three months ended March 31, 2024 and 2023, respectively, and were reclassified into earnings from other comprehensive income on a net-of-tax basis.

We have determined that unrealized losses at March 31, 2024, and December 31, 2023, primarily resulted from fluctuating interest rates, rather than a deterioration of the creditworthiness of the issuers. Therefore, we determined that an allowance for credit losses was not necessary. So long as we maintain the intent and ability to hold these securities to maturity, we are unlikely to experience realized 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, 2024:


	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	$	596	$	5	345	$	1,502	$	58	738
Mortgage-backed securities	154		2		137	485		42		267
Asset-backed securities	—		—		—	208		10		97
Municipal securities	—		—		—	112		7		110



Other	—		—		—	16		2		16

Total	$	750	$	7	482	$	2,323	$	119	1,228


	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	$	263	$	1	160	$	1,553	$	64	754
Mortgage-backed securities	123		2		98	549		42		283
Asset-backed securities	—		—		—	195		12		91
Municipal securities	—		—		—	117		6		116


Other	—		—		—	17		2		17

Total	$	386	$	3	258	$	2,431	$	126	1,261

Pursuant to the regulations governing our state health plan subsidiaries, we maintain statutory deposits and deposits required by government authorities primarily in cash, cash equivalents, U.S. Treasury securities, and corporate debt 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.

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. Such investments amounted to $261 million at March 31, 2024, of which $197 million will mature in one year or less, $59 million will mature in one through five years, and $5 million will mature after five years.