a) The following tables summarize the Company's available-for-sale investments. Commercial and residential mortgage-backed securities include securities issued by U.S. government-sponsored enterprises and U.S. government agencies. The net unrealized holding gains (losses) in the tables below are presented before taxes.


	March 31, 2024
(dollars in thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
Fixed maturity securities:
U.S. Treasury securities	$	4,113,319	$	12,079	$	(79,408)	$	4,045,990
U.S. government-sponsored enterprises	1,289,119		4,206		(99,430)		1,193,895
Obligations of states, municipalities and political subdivisions	4,041,845		10,059		(205,316)		3,846,588
Foreign governments	1,950,839		4,132		(126,930)		1,828,041
Commercial mortgage-backed securities	2,400,848		4,780		(137,472)		2,268,156
Residential mortgage-backed securities	463,710		155		(22,187)		441,678
Corporate bonds	968,072		5,424		(78,474)		895,022
Total fixed maturity securities	15,227,752		40,835		(749,217)		14,519,370
Short-term investments	2,618,162		597		(1,117)		2,617,642
Investments, available-for-sale	$	17,845,914	$	41,432	$	(750,334)	$	17,137,012


	December 31, 2023
(dollars in thousands)	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Estimated Fair Value
Fixed maturity securities:
U.S. Treasury securities	$	3,811,293	$	35,824	$	(62,404)	$	3,784,713
U.S. government-sponsored enterprises	1,225,426		7,292		(89,904)		1,142,814
Obligations of states, municipalities and political subdivisions	4,196,096		14,787		(181,578)		4,029,305
Foreign governments	1,858,845		21,450		(96,874)		1,783,421
Commercial mortgage-backed securities	2,371,406		8,605		(136,353)		2,243,658
Residential mortgage-backed securities	491,949		334		(21,861)		470,422
Corporate bonds	977,271		13,043		(71,915)		918,399
Total fixed maturity securities	14,932,286		101,335		(660,889)		14,372,732
Short-term investments	2,564,620		7,155		(393)		2,571,382
Investments, available-for-sale	$	17,496,906	$	108,490	$	(661,282)	$	16,944,114

b) The following tables summarize gross unrealized investment losses on available-for-sale investments by the length of time that securities have continuously been in an unrealized loss position. Unrealized losses on available-for-sale investments are typically the result of declines in the fair value of the investments due to increases in interest rates.


	March 31, 2024
	Less than 12 months				12 months or longer				Total
(dollars in thousands)	Estimated Fair Value		Gross Unrealized Holding Losses		Estimated Fair Value		Gross Unrealized Holding Losses		Estimated Fair Value		Gross Unrealized Holding Losses
Fixed maturity securities:
U.S. Treasury securities	$	1,553,613	$	(10,383)	$	1,388,322	$	(69,025)	$	2,941,935	$	(79,408)
U.S. government-sponsored enterprises	244,720		(3,629)		746,832		(95,801)		991,552		(99,430)
Obligations of states, municipalities and political subdivisions	920,869		(8,138)		2,376,770		(197,178)		3,297,639		(205,316)
Foreign governments	322,098		(3,059)		1,079,889		(123,871)		1,401,987		(126,930)
Commercial mortgage-backed securities	275,805		(4,101)		1,722,270		(133,371)		1,998,075		(137,472)
Residential mortgage-backed securities	25,905		(456)		401,845		(21,731)		427,750		(22,187)
Corporate bonds	148,340		(2,336)		574,746		(76,138)		723,086		(78,474)
Total fixed maturity securities	3,491,350		(32,102)		8,290,674		(717,115)		11,782,024		(749,217)
Short-term investments	2,272,608		(1,117)		—		—		2,272,608		(1,117)
Total	$	5,763,958	$	(33,219)	$	8,290,674	$	(717,115)	$	14,054,632	$	(750,334)

At March 31, 2024, the Company held 1,578 available-for-sale securities in an unrealized loss position with a total estimated fair value of $14.1 billion and gross unrealized losses of $750.3 million. Of these 1,578 securities, 1,126 securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $8.3 billion and gross unrealized losses of $717.1 million.


	December 31, 2023
	Less than 12 months				12 months or longer				Total
(dollars in thousands)	Estimated Fair Value		Gross Unrealized Holding Losses		Estimated Fair Value		Gross Unrealized Holding Losses		Estimated Fair Value		Gross Unrealized Holding Losses
Fixed maturity securities:
U.S. Treasury securities	$	317,027	$	(2,147)	$	1,507,784	$	(60,257)	$	1,824,811	$	(62,404)
U.S. government-sponsored enterprises	145,143		(2,134)		723,537		(87,770)		868,680		(89,904)
Obligations of states, municipalities and political subdivisions	679,124		(3,881)		2,332,281		(177,697)		3,011,405		(181,578)
Foreign governments	49,056		(128)		1,113,616		(96,746)		1,162,672		(96,874)
Commercial mortgage-backed securities	169,557		(1,792)		1,790,637		(134,561)		1,960,194		(136,353)
Residential mortgage-backed securities	20,420		(80)		431,705		(21,781)		452,125		(21,861)
Corporate bonds	34,340		(266)		615,501		(71,649)		649,841		(71,915)
Total fixed maturity securities	1,414,667		(10,428)		8,515,061		(650,461)		9,929,728		(660,889)
Short-term investments	52,601		(393)		—		—		52,601		(393)
Total	$	1,467,268	$	(10,821)	$	8,515,061	$	(650,461)	$	9,982,329	$	(661,282)

At December 31, 2023, the Company held 1,386 available-for-sale securities in an unrealized loss position with a total estimated fair value of $10.0 billion and gross unrealized losses of $661.3 million. Of these 1,386 securities, 1,131 securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $8.5 billion and gross unrealized losses of $650.5 million.

The Company completes a detailed analysis each quarter to assess whether the decline in the fair value of any investment below its cost basis is the result of a credit loss. All available-for-sale securities with unrealized losses are reviewed. The Company considers many factors in completing its quarterly review of securities with unrealized losses for credit-related impairment to determine whether a credit loss exists, including the extent to which fair value is below cost, the implied yield to maturity, rating downgrades of the security and whether or not the issuer has failed to make scheduled principal or interest payments. The Company also takes into consideration information about the financial condition of the issuer and industry factors that could negatively impact the issuer.

If the decline in fair value of an available-for-sale security below its amortized cost is considered to be the result of a credit loss, the Company compares the estimated present value of the cash flows expected to be collected to the amortized cost of the security. The extent to which the estimated present value of the cash flows expected to be collected is less than the amortized cost of the security represents the credit loss, which is recorded as an allowance and recognized in net income. The allowance is limited to the difference between the fair value and the amortized cost of the security. Any remaining decline in fair value represents the non-credit portion of the impairment, which is recognized in other comprehensive income. The Company did not have an allowance for credit losses for any available-for-sale securities as of March 31, 2024 or December 31, 2023.

Quarterly, the Company also considers whether it intends to sell an available-for-sale security or if it is more likely than not that it will be required to sell a security before recovery of its amortized cost. In these instances, a decline in fair value is recognized in net income based on the fair value of the security at the time of assessment, resulting in a new cost basis for the security.


(dollars in thousands)	Amortized Cost		Estimated Fair Value
Due in one year or less	$	1,108,711	$	1,091,274
Due after one year through five years	5,268,266		5,082,020
Due after five years through ten years	4,482,619		4,278,074
Due after ten years	1,503,598		1,358,168
	12,363,194		11,809,536
Commercial mortgage-backed securities	2,400,848		2,268,156
Residential mortgage-backed securities	463,710		441,678
Total fixed maturity securities	$	15,227,752	$	14,519,370


	Three Months Ended March 31,
(dollars in thousands)		2024		2023
Interest:
Fixed maturity securities		$	119,476	$	82,128
Short-term investments		32,184		26,619
Cash and cash equivalents and restricted cash and cash equivalents		39,078		27,601
Dividends on equity securities		32,693		27,482
		223,431		163,830
Investment expenses		(5,162)		(4,495)
Net investment income		$	218,269	$	159,335

e) The following table presents the components of net investment gains included in net income and the change in net unrealized losses included in other comprehensive income (loss). Gross realized investment gains and losses on fixed maturity securities, short-term investments and other investments were not material to the consolidated financial statements and are presented on a net basis in the following table.


	Three Months Ended March 31,
(dollars in thousands)		2024		2023
Fixed maturity securities, short-term investments and other investments:
Net realized investment losses		$	(4,488)	$	(3,221)
Equity securities:
Change in fair value of securities sold during the period		43		5,637
Change in fair value of securities held at the end of the period		906,726		370,147
Total change in fair value		906,769		375,784
Net investment gains		$	902,281	$	372,563
Change in net unrealized losses on available-for-sale investments included in other comprehensive income (loss):
Fixed maturity securities		$	(148,487)	$	209,179
Short-term investments		(7,282)		(810)
Net increase (decrease)		$	(155,769)	$	208,369