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:


	September 30, 2023
	Amortized Cost		Gross Unrealized				Estimated Fair Value
			Gains		Losses

	(In millions)
Corporate debt securities	$	2,739	$	1	$	121	$	2,619
Mortgage-backed securities	955		—		67		888
Asset-backed securities	378		—		17		361
Municipal securities	159		—		10		149
U.S. Treasury notes	51		—		—		51



Other	46		—		3		43
Total	$	4,328	$	1	$	218	$	4,111


	December 31, 2022
	Amortized Cost		Gross Unrealized				Estimated Fair Value
			Gains		Losses

	(In millions)
Corporate debt securities	$	2,303	$	2	$	121	$	2,184
Mortgage-backed securities	787		—		56		731
Asset-backed securities	308		—		20		288
Municipal securities	160		—		11		149
U.S. Treasury notes	106		—		1		105


Other	45		—		3		42
Total	$	3,709	$	2	$	212	$	3,499


	Amortized Cost		Estimated Fair Value

	(In millions)
Due in one year or less	$	474	$	464
Due after one year through five years	2,447		2,334
Due after five years through ten years	440		424
Due after ten years	967		889
Total	$	4,328	$	4,111

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 nine months ended September 30, 2023, and 2022. Gross realized investment losses amounted to $10 million in nine months ended September 30, 2023, and were reclassified into earnings from other comprehensive income on a net-of-tax basis. Gross realized investment losses were insignificant in nine months ended September 30, 2022.

We have determined that unrealized losses at September 30, 2023, and December 31, 2022, 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 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 September 30, 2023:


	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	$	942	$	23	549	$	1,559	$	98	785
Mortgage-backed securities	272		10		219	567		57		297
Asset-backed securities	136		1		96	206		16		100
Municipal securities	—		—		—	121		10		123

Other	13		1		16	17		2		16



Total	$	1,363	$	35	880	$	2,470	$	183	1,321


	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	$	1,124	$	45	683	$	887	$	76	371
Mortgage-backed securities	395		20		220	319		36		131
Asset-backed securities	161		6		108	118		14		59
Municipal securities	75		4		83	57		7		57
U.S. Treasury notes	88		1		6	—		—		—

Other	15		1		16	17		2		6

Total	$	1,858	$	77	1,116	$	1,398	$	135	624

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 September 30, 2023, of which $100 million will mature in one year or less, $155 million will mature in one through five years, and $6 million will mature after five years.