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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, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	$ 1,467,268	$ 7,924,623
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(10,821)	(568,299)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	8,515,061	3,356,273
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(650,461)	(393,911)
Available-for-sale fixed maturity securities, estimated fair value	9,982,329	11,280,896
Available-for-sale fixed maturity securities, gross unrealized holding losses	(661,282)	(962,210)
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months	52,601	774,480
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(393)	(58)
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	52,601	774,480
Short-term investments, available-for-sale, unrealized loss position, accumulated loss	(393)	(58)
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	317,027	735,605
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(2,147)	(30,583)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,507,784	1,907,922
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(60,257)	(107,910)
Available-for-sale fixed maturity securities, estimated fair value	1,824,811	2,643,527
Available-for-sale fixed maturity securities, gross unrealized holding losses	(62,404)	(138,493)
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	145,143	413,495
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(2,134)	(40,488)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	723,537	331,391
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(87,770)	(65,591)
Available-for-sale fixed maturity securities, estimated fair value	868,680	744,886
Available-for-sale fixed maturity securities, gross unrealized holding losses	(89,904)	(106,079)
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	679,124	2,474,289
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(3,881)	(164,537)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	2,332,281	348,943
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(177,697)	(82,694)
Available-for-sale fixed maturity securities, estimated fair value	3,011,405	2,823,232
Available-for-sale fixed maturity securities, gross unrealized holding losses	(181,578)	(247,231)
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	49,056	900,322
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(128)	(115,324)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,113,616	300,423
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(96,746)	(54,399)
Available-for-sale fixed maturity securities, estimated fair value	1,162,672	1,200,745
Available-for-sale fixed maturity securities, gross unrealized holding losses	(96,874)	(169,723)
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	169,557	1,611,603
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(1,792)	(117,482)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	1,790,637	305,217
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(134,561)	(52,186)
Available-for-sale fixed maturity securities, estimated fair value	1,960,194	1,916,820
Available-for-sale fixed maturity securities, gross unrealized holding losses	(136,353)	(169,668)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	20,420	516,423
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(80)	(25,232)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	431,705	9,342
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(21,781)	(1,572)
Available-for-sale fixed maturity securities, estimated fair value	452,125	525,765
Available-for-sale fixed maturity securities, gross unrealized holding losses	(21,861)	(26,804)
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	34,340	498,406
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(266)	(74,595)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	615,501	153,035
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(71,649)	(29,559)
Available-for-sale fixed maturity securities, estimated fair value	649,841	651,441
Available-for-sale fixed maturity securities, gross unrealized holding losses	(71,915)	(104,154)
Total fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months	1,414,667	7,150,143
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months	(10,428)	(568,241)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer	8,515,061	3,356,273
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer	(650,461)	(393,911)
Available-for-sale fixed maturity securities, estimated fair value	9,929,728	10,506,416
Available-for-sale fixed maturity securities, gross unrealized holding losses	$ (660,889)	$ (962,152)

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