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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, 2025 | Dec. 31, 2024
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Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | $ 1,385,754 | $ 5,122,368
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (7,590) | (109,879)
Available-for-sale securities, estimated fair value, 12 months or longer | 6,627,517 | 7,448,033
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (353,511) | (633,124)
Available-for-sale securities, estimated fair value | 8,013,271 | 12,570,401
Available-for-sale securities, gross unrealized holding losses | (361,101) | (743,003)
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 382,948 | 1,593,711
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (954) | (27,213)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,102,470 | 1,444,869
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (16,550) | (41,256)
Available-for-sale securities, estimated fair value | 1,485,418 | 3,038,580
Available-for-sale securities, gross unrealized holding losses | (17,504) | (68,469)
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 72,182 | 415,333
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (493) | (10,938)
Available-for-sale securities, estimated fair value, 12 months or longer | 735,876 | 691,781
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (61,428) | (90,973)
Available-for-sale securities, estimated fair value | 808,058 | 1,107,114
Available-for-sale securities, gross unrealized holding losses | (61,921) | (101,911)
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 | 173,841 | 1,133,275
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (1,119) | (21,242)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,897,264 | 2,024,298
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (112,175) | (178,278)
Available-for-sale securities, estimated fair value | 2,071,105 | 3,157,573
Available-for-sale securities, gross unrealized holding losses | (113,294) | (199,520)
Foreign governments, agencies and supranationals [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 227,228 | 1,056,877
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (3,597) | (29,890)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,206,635 | 1,246,215
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (75,969) | (177,412)
Available-for-sale securities, estimated fair value | 1,433,863 | 2,303,092
Available-for-sale securities, gross unrealized holding losses | (79,566) | (207,302)
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 122,647 | 757,562
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (723) | (13,880)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,442,223 | 1,710,436
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (66,630) | (109,992)
Available-for-sale securities, estimated fair value | 1,564,870 | 2,467,998
Available-for-sale securities, gross unrealized holding losses | (67,353) | (123,872)
Non-agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 0 | 0
Available-for-sale securities, gross unrealized holding losses, less than 12 months | 0 | 0
Available-for-sale securities, estimated fair value, 12 months or longer | 61,152 | 118,030
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (703) | (4,343)
Available-for-sale securities, estimated fair value | 61,152 | 118,030
Available-for-sale securities, gross unrealized holding losses | (703) | (4,343)
Corporate and university bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 14,429 | 2,107
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (254) | (137)
Available-for-sale securities, estimated fair value, 12 months or longer | 181,897 | 212,404
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (20,056) | (30,870)
Available-for-sale securities, estimated fair value | 196,326 | 214,511
Available-for-sale securities, gross unrealized holding losses | (20,310) | (31,007)
Total fixed maturity securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 993,275 | 4,958,865
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (7,140) | (103,300)
Available-for-sale securities, estimated fair value, 12 months or longer | 6,627,517 | 7,448,033
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (353,511) | (633,124)
Available-for-sale securities, estimated fair value | 7,620,792 | 12,406,898
Available-for-sale securities, gross unrealized holding losses | (360,651) | (736,424)
Short-term investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Short-term investments, estimated fair value, less than 12 months | 392,479 | 163,503
Short-term investments, gross unrealized holding losses, less than 12 months | (450) | (6,579)
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 | 392,479 | 163,503
Short-term investments, gross unrealized holding losses | $ (450) | $ (6,579)

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