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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, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	$ 5,122,368	$ 1,467,268
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(109,879)	(10,821)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	7,448,033	8,515,061
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(633,124)	(650,461)
Available-for-sale fixed maturity securities, estimated fair value	12,570,401	9,982,329
Available-for-sale fixed maturity securities, gross unrealized holding losses	(743,003)	(661,282)
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months	163,503	52,601
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(6,579)	(393)
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	163,503	52,601
Short-term investments, available-for-sale, unrealized loss position, accumulated loss	(6,579)	(393)
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	1,593,711	317,027
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(27,213)	(2,147)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,444,869	1,507,784
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(41,256)	(60,257)
Available-for-sale fixed maturity securities, estimated fair value	3,038,580	1,824,811
Available-for-sale fixed maturity securities, gross unrealized holding losses	(68,469)	(62,404)
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	415,333	145,143
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(10,938)	(2,134)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	691,781	723,537
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(90,973)	(87,770)
Available-for-sale fixed maturity securities, estimated fair value	1,107,114	868,680
Available-for-sale fixed maturity securities, gross unrealized holding losses	(101,911)	(89,904)
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	1,133,275	679,124
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(21,242)	(3,881)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	2,024,298	2,332,281
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(178,278)	(177,697)
Available-for-sale fixed maturity securities, estimated fair value	3,157,573	3,011,405
Available-for-sale fixed maturity securities, gross unrealized holding losses	(199,520)	(181,578)
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	1,056,877	83,396
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(29,890)	(394)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,246,215	1,492,912
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(177,412)	(141,614)
Available-for-sale fixed maturity securities, estimated fair value	2,303,092	1,576,308
Available-for-sale fixed maturity securities, gross unrealized holding losses	(207,302)	(142,008)
Agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	757,562	189,977
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(13,880)	(1,872)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,710,436	1,889,272
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(109,992)	(133,739)
Available-for-sale fixed maturity securities, estimated fair value	2,467,998	2,079,249
Available-for-sale fixed maturity securities, gross unrealized holding losses	(123,872)	(135,611)
Non-agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	0	0
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	0	0
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	118,030	333,070
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(4,343)	(22,603)
Available-for-sale fixed maturity securities, estimated fair value	118,030	333,070
Available-for-sale fixed maturity securities, gross unrealized holding losses	(4,343)	(22,603)
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	2,107	0
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(137)	0
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	212,404	236,205
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(30,870)	(26,781)
Available-for-sale fixed maturity securities, estimated fair value	214,511	236,205
Available-for-sale fixed maturity securities, gross unrealized holding losses	(31,007)	(26,781)
Total fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	4,958,865	1,414,667
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(103,300)	(10,428)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	7,448,033	8,515,061
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(633,124)	(650,461)
Available-for-sale fixed maturity securities, estimated fair value	12,406,898	9,929,728
Available-for-sale fixed maturity securities, gross unrealized holding losses	$ (736,424)	$ (660,889)

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