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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	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$ 5,117,903	$ 7,924,623
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(72,501)	(568,299)
Available-for-sale securities, estimated fair value, 12 months or longer	8,111,873	3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(844,824)	(393,911)
Available-for-sale securities, estimated fair value	13,229,776	11,280,896
Available-for-sale securities, gross unrealized holding losses	(917,325)	(962,210)
Short-term investments, estimated fair value, less than 12 months	1,324,927	774,480
Short-term investments, gross unrealized holding losses, less than 12 months	(1,526)	(58)
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	1,324,927	774,480
Short-term investments, gross unrealized holding losses	(1,526)	(58)
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	1,072,516	735,605
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(17,213)	(30,583)
Available-for-sale securities, estimated fair value, 12 months or longer	1,956,782	1,907,922
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(116,329)	(107,910)
Available-for-sale securities, estimated fair value	3,029,298	2,643,527
Available-for-sale securities, gross unrealized holding losses	(133,542)	(138,493)
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	301,249	413,495
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(6,205)	(40,488)
Available-for-sale securities, estimated fair value, 12 months or longer	628,588	331,391
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(100,007)	(65,591)
Available-for-sale securities, estimated fair value	929,837	744,886
Available-for-sale securities, gross unrealized holding losses	(106,212)	(106,079)
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	1,434,351	2,474,289
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(18,517)	(164,537)
Available-for-sale securities, estimated fair value, 12 months or longer	1,929,349	348,943
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(201,591)	(82,694)
Available-for-sale securities, estimated fair value	3,363,700	2,823,232
Available-for-sale securities, gross unrealized holding losses	(220,108)	(247,231)
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	359,569	900,322
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(12,927)	(115,324)
Available-for-sale securities, estimated fair value, 12 months or longer	1,005,886	300,423
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(138,623)	(54,399)
Available-for-sale securities, estimated fair value	1,365,455	1,200,745
Available-for-sale securities, gross unrealized holding losses	(151,550)	(169,723)
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	419,295	1,611,603
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(12,357)	(117,482)
Available-for-sale securities, estimated fair value, 12 months or longer	1,634,450	305,217
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(165,900)	(52,186)
Available-for-sale securities, estimated fair value	2,053,745	1,916,820
Available-for-sale securities, gross unrealized holding losses	(178,257)	(169,668)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	115,044	516,423
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(2,216)	(25,232)
Available-for-sale securities, estimated fair value, 12 months or longer	364,356	9,342
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(26,557)	(1,572)
Available-for-sale securities, estimated fair value	479,400	525,765
Available-for-sale securities, gross unrealized holding losses	(28,773)	(26,804)
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	0	1,640
Available-for-sale securities, gross unrealized holding losses, less than 12 months	0	(53)
Available-for-sale securities, estimated fair value, 12 months or longer	1,204	0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(37)	0
Available-for-sale securities, estimated fair value	1,204	1,640
Available-for-sale securities, gross unrealized holding losses	(37)	(53)
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	90,952	496,766
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(1,540)	(74,542)
Available-for-sale securities, estimated fair value, 12 months or longer	591,258	153,035
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(95,780)	(29,559)
Available-for-sale securities, estimated fair value	682,210	649,801
Available-for-sale securities, gross unrealized holding losses	(97,320)	(104,101)
Total fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	3,792,976	7,150,143
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(70,975)	(568,241)
Available-for-sale securities, estimated fair value, 12 months or longer	8,111,873	3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(844,824)	(393,911)
Available-for-sale securities, estimated fair value	11,904,849	10,506,416
Available-for-sale securities, gross unrealized holding losses	$ (915,799)	$ (962,152)

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