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Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
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Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | $ 5,763,958 | $ 1,467,268
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (33,219) | (10,821)
Available-for-sale securities, estimated fair value, 12 months or longer | 8,290,674 | 8,515,061
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (717,115) | (650,461)
Available-for-sale securities, estimated fair value | 14,054,632 | 9,982,329
Available-for-sale securities, gross unrealized holding losses | (750,334) | (661,282)
Short-term investments, estimated fair value, less than 12 months | 2,272,608 | 52,601
Short-term investments, gross unrealized holding losses, less than 12 months | (1,117) | (393)
Short-term investments, estimated fair value, 12 months or longer | 0 | 0
Short-term investments, gross unrealized holding losses, 12 months or longer | 0 | 0
Short-term investments, estimated fair value | 2,272,608 | 52,601
Short-term investments, gross unrealized holding losses | (1,117) | (393)
Fixed maturities, available-for-sale, amortized cost | 15,227,752 | 14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains | 40,835 | 101,335
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (749,217) | (660,889)
Available-for-sale fixed maturity investments, estimated fair value | 14,519,370 | 14,372,732
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 1,553,613 | 317,027
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (10,383) | (2,147)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,388,322 | 1,507,784
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (69,025) | (60,257)
Available-for-sale securities, estimated fair value | 2,941,935 | 1,824,811
Available-for-sale securities, gross unrealized holding losses | (79,408) | (62,404)
Fixed maturities, available-for-sale, amortized cost | 4,113,319 | 3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains | 12,079 | 35,824
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (79,408) | (62,404)
Available-for-sale fixed maturity investments, estimated fair value | 4,045,990 | 3,784,713
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 244,720 | 145,143
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (3,629) | (2,134)
Available-for-sale securities, estimated fair value, 12 months or longer | 746,832 | 723,537
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (95,801) | (87,770)
Available-for-sale securities, estimated fair value | 991,552 | 868,680
Available-for-sale securities, gross unrealized holding losses | (99,430) | (89,904)
Fixed maturities, available-for-sale, amortized cost | 1,289,119 | 1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,206 | 7,292
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (99,430) | (89,904)
Available-for-sale fixed maturity investments, estimated fair value | 1,193,895 | 1,142,814
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 920,869 | 679,124
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (8,138) | (3,881)
Available-for-sale securities, estimated fair value, 12 months or longer | 2,376,770 | 2,332,281
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (197,178) | (177,697)
Available-for-sale securities, estimated fair value | 3,297,639 | 3,011,405
Available-for-sale securities, gross unrealized holding losses | (205,316) | (181,578)
Fixed maturities, available-for-sale, amortized cost | 4,041,845 | 4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains | 10,059 | 14,787
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (205,316) | (181,578)
Available-for-sale fixed maturity investments, estimated fair value | 3,846,588 | 4,029,305
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 322,098 | 49,056
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (3,059) | (128)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,079,889 | 1,113,616
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (123,871) | (96,746)
Available-for-sale securities, estimated fair value | 1,401,987 | 1,162,672
Available-for-sale securities, gross unrealized holding losses | (126,930) | (96,874)
Fixed maturities, available-for-sale, amortized cost | 1,950,839 | 1,858,845
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,132 | 21,450
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (126,930) | (96,874)
Available-for-sale fixed maturity investments, estimated fair value | 1,828,041 | 1,783,421
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 275,805 | 169,557
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (4,101) | (1,792)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,722,270 | 1,790,637
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (133,371) | (134,561)
Available-for-sale securities, estimated fair value | 1,998,075 | 1,960,194
Available-for-sale securities, gross unrealized holding losses | (137,472) | (136,353)
Fixed maturities, available-for-sale, amortized cost | 2,400,848 | 2,371,406
Available-for-sale fixed maturity investments, gross unrealized holding gains | 4,780 | 8,605
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (137,472) | (136,353)
Available-for-sale fixed maturity investments, estimated fair value | 2,268,156 | 2,243,658
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 25,905 | 20,420
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (456) | (80)
Available-for-sale securities, estimated fair value, 12 months or longer | 401,845 | 431,705
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (21,731) | (21,781)
Available-for-sale securities, estimated fair value | 427,750 | 452,125
Available-for-sale securities, gross unrealized holding losses | (22,187) | (21,861)
Fixed maturities, available-for-sale, amortized cost | 463,710 | 491,949
Available-for-sale fixed maturity investments, gross unrealized holding gains | 155 | 334
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (22,187) | (21,861)
Available-for-sale fixed maturity investments, estimated fair value | 441,678 | 470,422
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 148,340 | 34,340
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (2,336) | (266)
Available-for-sale securities, estimated fair value, 12 months or longer | 574,746 | 615,501
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (76,138) | (71,649)
Available-for-sale securities, estimated fair value | 723,086 | 649,841
Available-for-sale securities, gross unrealized holding losses | (78,474) | (71,915)
Fixed maturities, available-for-sale, amortized cost | 968,072 | 977,271
Available-for-sale fixed maturity investments, gross unrealized holding gains | 5,424 | 13,043
Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax | (78,474) | (71,915)
Available-for-sale fixed maturity investments, estimated fair value | 895,022 | 918,399
Total fixed maturities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 3,491,350 | 1,414,667
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (32,102) | (10,428)
Available-for-sale securities, estimated fair value, 12 months or longer | 8,290,674 | 8,515,061
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (717,115) | (650,461)
Available-for-sale securities, estimated fair value | 11,782,024 | 9,929,728
Available-for-sale securities, gross unrealized holding losses | $ (749,217) | $ (660,889)

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