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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, 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 | $ 4,153,208 | $ 5,122,368
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (32,678) | (109,879)
Available-for-sale securities, estimated fair value, 12 months or longer | 7,244,968 | 7,448,033
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (514,200) | (633,124)
Available-for-sale securities, estimated fair value | 11,398,176 | 12,570,401
Available-for-sale securities, gross unrealized holding losses | (546,878) | (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 | 683,847 | 1,593,711
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (4,986) | (27,213)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,277,393 | 1,444,869
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (28,576) | (41,256)
Available-for-sale securities, estimated fair value | 1,961,240 | 3,038,580
Available-for-sale securities, gross unrealized holding losses | (33,562) | (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 | 243,333 | 415,333
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (3,966) | (10,938)
Available-for-sale securities, estimated fair value, 12 months or longer | 713,889 | 691,781
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (73,587) | (90,973)
Available-for-sale securities, estimated fair value | 957,222 | 1,107,114
Available-for-sale securities, gross unrealized holding losses | (77,553) | (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 | 598,623 | 1,133,275
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (6,410) | (21,242)
Available-for-sale securities, estimated fair value, 12 months or longer | 2,050,569 | 2,024,298
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (147,577) | (178,278)
Available-for-sale securities, estimated fair value | 2,649,192 | 3,157,573
Available-for-sale securities, gross unrealized holding losses | (153,987) | (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 | 585,664 | 1,056,877
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (11,664) | (29,890)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,268,147 | 1,246,215
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (149,136) | (177,412)
Available-for-sale securities, estimated fair value | 1,853,811 | 2,303,092
Available-for-sale securities, gross unrealized holding losses | (160,800) | (207,302)
Agency mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale securities, estimated fair value, less than 12 months | 162,695 | 757,562
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (3,264) | (13,880)
Available-for-sale securities, estimated fair value, 12 months or longer | 1,631,014 | 1,710,436
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (87,733) | (109,992)
Available-for-sale securities, estimated fair value | 1,793,709 | 2,467,998
Available-for-sale securities, gross unrealized holding losses | (90,997) | (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 | 101,954 | 118,030
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (2,783) | (4,343)
Available-for-sale securities, estimated fair value | 101,954 | 118,030
Available-for-sale securities, gross unrealized holding losses | (2,783) | (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 | 0 | 2,107
Available-for-sale securities, gross unrealized holding losses, less than 12 months | 0 | (137)
Available-for-sale securities, estimated fair value, 12 months or longer | 202,002 | 212,404
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (24,808) | (30,870)
Available-for-sale securities, estimated fair value | 202,002 | 214,511
Available-for-sale securities, gross unrealized holding losses | (24,808) | (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 | 2,274,162 | 4,958,865
Available-for-sale securities, gross unrealized holding losses, less than 12 months | (30,290) | (103,300)
Available-for-sale securities, estimated fair value, 12 months or longer | 7,244,968 | 7,448,033
Available-for-sale securities, gross unrealized holding losses, 12 months or longer | (514,200) | (633,124)
Available-for-sale securities, estimated fair value | 9,519,130 | 12,406,898
Available-for-sale securities, gross unrealized holding losses | (544,490) | (736,424)
Short-term investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Short-term investments, estimated fair value, less than 12 months | 1,879,046 | 163,503
Short-term investments, gross unrealized holding losses, less than 12 months | (2,388) | (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 | 1,879,046 | 163,503
Short-term investments, gross unrealized holding losses | $ (2,388) | $ (6,579)

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