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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 | Dec. 31, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | $ 1,467,268 | $ 7,924,623
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (10,821) | (568,299)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 8,515,061 | 3,356,273
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (650,461) | (393,911)
Available-for-sale fixed maturity securities, estimated fair value | 9,982,329 | 11,280,896
Available-for-sale fixed maturity securities, gross unrealized holding losses | (661,282) | (962,210)
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months | 52,601 | 774,480
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss | (393) | (58)
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer | 0 | 0
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss | 0 | 0
Short-term investments, available-for-sale, unrealized loss position, total | 52,601 | 774,480
Short-term investments, available-for-sale, unrealized loss position, accumulated loss | (393) | (58)
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 317,027 | 735,605
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (2,147) | (30,583)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 1,507,784 | 1,907,922
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (60,257) | (107,910)
Available-for-sale fixed maturity securities, estimated fair value | 1,824,811 | 2,643,527
Available-for-sale fixed maturity securities, gross unrealized holding losses | (62,404) | (138,493)
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 145,143 | 413,495
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (2,134) | (40,488)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 723,537 | 331,391
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (87,770) | (65,591)
Available-for-sale fixed maturity securities, estimated fair value | 868,680 | 744,886
Available-for-sale fixed maturity securities, gross unrealized holding losses | (89,904) | (106,079)
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 679,124 | 2,474,289
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (3,881) | (164,537)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 2,332,281 | 348,943
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (177,697) | (82,694)
Available-for-sale fixed maturity securities, estimated fair value | 3,011,405 | 2,823,232
Available-for-sale fixed maturity securities, gross unrealized holding losses | (181,578) | (247,231)
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 49,056 | 900,322
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (128) | (115,324)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 1,113,616 | 300,423
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (96,746) | (54,399)
Available-for-sale fixed maturity securities, estimated fair value | 1,162,672 | 1,200,745
Available-for-sale fixed maturity securities, gross unrealized holding losses | (96,874) | (169,723)
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 169,557 | 1,611,603
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (1,792) | (117,482)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 1,790,637 | 305,217
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (134,561) | (52,186)
Available-for-sale fixed maturity securities, estimated fair value | 1,960,194 | 1,916,820
Available-for-sale fixed maturity securities, gross unrealized holding losses | (136,353) | (169,668)
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 20,420 | 516,423
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (80) | (25,232)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 431,705 | 9,342
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (21,781) | (1,572)
Available-for-sale fixed maturity securities, estimated fair value | 452,125 | 525,765
Available-for-sale fixed maturity securities, gross unrealized holding losses | (21,861) | (26,804)
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 34,340 | 498,406
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (266) | (74,595)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 615,501 | 153,035
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (71,649) | (29,559)
Available-for-sale fixed maturity securities, estimated fair value | 649,841 | 651,441
Available-for-sale fixed maturity securities, gross unrealized holding losses | (71,915) | (104,154)
Total fixed maturities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months | 1,414,667 | 7,150,143
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months | (10,428) | (568,241)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer | 8,515,061 | 3,356,273
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer | (650,461) | (393,911)
Available-for-sale fixed maturity securities, estimated fair value | 9,929,728 | 10,506,416
Available-for-sale fixed maturity securities, gross unrealized holding losses | $ (660,889) | $ (962,152)

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