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Investments (Tables)
 3 Months Ended 12 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]    
Available-For-Sale Investments 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 enterprises1,289,119 4,206 (99,430)1,193,895 
Obligations of states, municipalities and political subdivisions4,041,845 10,059 (205,316)3,846,588 
Foreign governments1,950,839 4,132 (126,930)1,828,041 
Commercial mortgage-backed securities2,400,848 4,780 (137,472)2,268,156 
Residential mortgage-backed securities463,710 155 (22,187)441,678 
Corporate bonds968,072 5,424 (78,474)895,022 
Total fixed maturity securities15,227,752 40,835 (749,217)14,519,370 
Short-term investments2,618,162 597 (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 enterprises1,225,426 7,292 (89,904)1,142,814 
Obligations of states, municipalities and political subdivisions4,196,096 14,787 (181,578)4,029,305 
Foreign governments1,858,845 21,450 (96,874)1,783,421 
Commercial mortgage-backed securities2,371,406 8,605 (136,353)2,243,658 
Residential mortgage-backed securities491,949 334 (21,861)470,422 
Corporate bonds977,271 13,043 (71,915)918,399 
Total fixed maturity securities14,932,286 101,335 (660,889)14,372,732 
Short-term investments2,564,620 7,155 (393)2,571,382 
Investments, available-for-sale$17,496,906 $108,490 $(661,282)$16,944,114 
Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position 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 months12 months or longerTotal
(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 enterprises244,720 (3,629)746,832 (95,801)991,552 (99,430)
Obligations of states, municipalities and political subdivisions920,869 (8,138)2,376,770 (197,178)3,297,639 (205,316)
Foreign governments322,098 (3,059)1,079,889 (123,871)1,401,987 (126,930)
Commercial mortgage-backed securities275,805 (4,101)1,722,270 (133,371)1,998,075 (137,472)
Residential mortgage-backed securities25,905 (456)401,845 (21,731)427,750 (22,187)
Corporate bonds148,340 (2,336)574,746 (76,138)723,086 (78,474)
Total fixed maturity securities3,491,350 (32,102)8,290,674 (717,115)11,782,024 (749,217)
Short-term investments2,272,608 (1,117)— — 2,272,608 (1,117)
Total$5,763,958 $(33,219)$8,290,674 $(717,115)$14,054,632 $(750,334)
December 31, 2023
Less than 12 months12 months or longerTotal
(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 enterprises145,143 (2,134)723,537 (87,770)868,680 (89,904)
Obligations of states, municipalities and political subdivisions679,124 (3,881)2,332,281 (177,697)3,011,405 (181,578)
Foreign governments49,056 (128)1,113,616 (96,746)1,162,672 (96,874)
Commercial mortgage-backed securities169,557 (1,792)1,790,637 (134,561)1,960,194 (136,353)
Residential mortgage-backed securities20,420 (80)431,705 (21,781)452,125 (21,861)
Corporate bonds34,340 (266)615,501 (71,649)649,841 (71,915)
Total fixed maturity securities1,414,667 (10,428)8,515,061 (650,461)9,929,728 (660,889)
Short-term investments52,601 (393)— — 52,601 (393)
Total$1,467,268 $(10,821)$8,515,061 $(650,461)$9,982,329 $(661,282)
Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturity Securities By Contractual Maturity Date The amortized cost and estimated fair value of fixed maturity securities at March 31, 2024 are shown below by contractual maturity.
(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 years5,268,266 5,082,020 
Due after five years through ten years4,482,619 4,278,074 
Due after ten years1,503,598 1,358,168 
12,363,194 11,809,536 
Commercial mortgage-backed securities2,400,848 2,268,156 
Residential mortgage-backed securities463,710 441,678 
Total fixed maturity securities$15,227,752 $14,519,370 
  
Components Of Net Investment Income The following table presents the components of net investment income.
Three Months Ended March 31,
(dollars in thousands)20242023
Interest:
Fixed maturity securities$119,476 $82,128 
Short-term investments32,184 26,619 
Cash and cash equivalents and restricted cash and cash equivalents
39,078 27,601 
Dividends on equity securities32,693 27,482 
223,431 163,830 
Investment expenses(5,162)(4,495)
Net investment income
$218,269 $159,335 
  
Summary Of Net Investment Gains (Losses) And The Change In Net Unrealized Gains On Available-For-Sale Investments 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)20242023
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 period43 5,637 
Change in fair value of securities held at the end of the period906,726 370,147 
Total change in fair value906,769 375,784 
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