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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 | Sep. 30, 2023 | Dec. 31, 2022
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Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | $ 4,222,002 | $ 7,924,623
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (120,380) | (568,299)
Available-for-sale securities, estimated fair value, 12 months or longer | 8,805,736 | 3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (1,001,104) | (393,911)
Available-for-sale securities, estimated fair value | 13,027,738 | 11,280,896
Available-for-sale securities, gross unrealized holding losses | (1,121,484) | (962,210)
Short-term investments, estimated fair value, less than 12 months | 203,710 | 774,480
Short-term investments, gross unrealized holding losses, less than 12 months | (746) | (58)
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 | 203,710 | 774,480
Short-term investments, gross unrealized holding losses | (746) | (58)
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 1,339,577 | 735,605
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (33,496) | (30,583)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,971,252 | 1,907,922
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (96,666) | (107,910)
Available-for-sale securities, estimated fair value | 3,310,829 | 2,643,527
Available-for-sale securities, gross unrealized holding losses | (130,162) | (138,493)
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 302,362 | 413,495
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (13,784) | (40,488)
Available-for-sale securities, estimated fair value, 12 months or longer | 693,322 | 331,391
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (123,949) | (65,591)
Available-for-sale securities, estimated fair value | 995,684 | 744,886
Available-for-sale securities, gross unrealized holding losses | (137,733) | (106,079)
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 | 1,469,105 | 2,474,289
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (36,622) | (164,537)
Available-for-sale securities, estimated fair value, 12 months or longer | 2,240,788 | 348,943
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (271,562) | (82,694)
Available-for-sale securities, estimated fair value | 3,709,893 | 2,823,232
Available-for-sale securities, gross unrealized holding losses | (308,184) | (247,231)
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 442,453 | 900,322
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (18,214) | (115,324)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,095,516 | 300,423
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (170,914) | (54,399)
Available-for-sale securities, estimated fair value | 1,537,969 | 1,200,745
Available-for-sale securities, gross unrealized holding losses | (189,128) | (169,723)
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 321,255 | 1,611,603
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (13,554) | (117,482)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,764,367 | 305,217
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (194,912) | (52,186)
Available-for-sale securities, estimated fair value | 2,085,622 | 1,916,820
Available-for-sale securities, gross unrealized holding losses | (208,466) | (169,668)
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 45,081 | 516,423
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (1,074) | (25,232)
Available-for-sale securities, estimated fair value, 12 months or longer | 433,695 | 9,342
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (31,612) | (1,572)
Available-for-sale securities, estimated fair value | 478,776 | 525,765
Available-for-sale securities, gross unrealized holding losses | (32,686) | (26,804)
Asset-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 0 | 1,640
Available-for-sale securities, gross unrealized holding losses, less than 12 months | 0 | (53)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,198 | 0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (28) | 0
Available-for-sale securities, estimated fair value | 1,198 | 1,640
Available-for-sale securities, gross unrealized holding losses | (28) | (53)
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 98,459 | 496,766
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (2,890) | (74,542)
Available-for-sale securities, estimated fair value, 12 months or longer | 605,598 | 153,035
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (111,461) | (29,559)
Available-for-sale securities, estimated fair value | 704,057 | 649,801
Available-for-sale securities, gross unrealized holding losses | (114,351) | (104,101)
Total fixed maturities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 4,018,292 | 7,150,143
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (119,634) | (568,241)
Available-for-sale securities, estimated fair value, 12 months or longer | 8,805,736 | 3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (1,001,104) | (393,911)
Available-for-sale securities, estimated fair value | 12,824,028 | 10,506,416
Available-for-sale securities, gross unrealized holding losses | $ (1,120,738) | $ (962,152)

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