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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
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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