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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, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$ 4,222,002	$ 7,924,623
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(120,380)	(568,299)
Available-for-sale securities, estimated fair value, 12 months or longer	8,805,736	3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(1,001,104)	(393,911)
Available-for-sale securities, estimated fair value	13,027,738	11,280,896
Available-for-sale securities, gross unrealized holding losses	(1,121,484)	(962,210)
Short-term investments, estimated fair value, less than 12 months	203,710	774,480
Short-term investments, gross unrealized holding losses, less than 12 months	(746)	(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	203,710	774,480
Short-term investments, gross unrealized holding losses	(746)	(58)
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	1,339,577	735,605
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(33,496)	(30,583)
Available-for-sale securities, estimated fair value, 12 months or longer	1,971,252	1,907,922
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(96,666)	(107,910)
Available-for-sale securities, estimated fair value	3,310,829	2,643,527
Available-for-sale securities, gross unrealized holding losses	(130,162)	(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	302,362	413,495
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(13,784)	(40,488)
Available-for-sale securities, estimated fair value, 12 months or longer	693,322	331,391
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(123,949)	(65,591)
Available-for-sale securities, estimated fair value	995,684	744,886
Available-for-sale securities, gross unrealized holding losses	(137,733)	(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	1,469,105	2,474,289
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(36,622)	(164,537)
Available-for-sale securities, estimated fair value, 12 months or longer	2,240,788	348,943
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(271,562)	(82,694)
Available-for-sale securities, estimated fair value	3,709,893	2,823,232
Available-for-sale securities, gross unrealized holding losses	(308,184)	(247,231)
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	442,453	900,322
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(18,214)	(115,324)
Available-for-sale securities, estimated fair value, 12 months or longer	1,095,516	300,423
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(170,914)	(54,399)
Available-for-sale securities, estimated fair value	1,537,969	1,200,745
Available-for-sale securities, gross unrealized holding losses	(189,128)	(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	321,255	1,611,603
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(13,554)	(117,482)
Available-for-sale securities, estimated fair value, 12 months or longer	1,764,367	305,217
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(194,912)	(52,186)
Available-for-sale securities, estimated fair value	2,085,622	1,916,820
Available-for-sale securities, gross unrealized holding losses	(208,466)	(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	45,081	516,423
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(1,074)	(25,232)
Available-for-sale securities, estimated fair value, 12 months or longer	433,695	9,342
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(31,612)	(1,572)
Available-for-sale securities, estimated fair value	478,776	525,765
Available-for-sale securities, gross unrealized holding losses	(32,686)	(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	1,198	0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(28)	0
Available-for-sale securities, estimated fair value	1,198	1,640
Available-for-sale securities, gross unrealized holding losses	(28)	(53)
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	98,459	496,766
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(2,890)	(74,542)
Available-for-sale securities, estimated fair value, 12 months or longer	605,598	153,035
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(111,461)	(29,559)
Available-for-sale securities, estimated fair value	704,057	649,801
Available-for-sale securities, gross unrealized holding losses	(114,351)	(104,101)
Total fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	4,018,292	7,150,143
Available-for-sale securities, gross unrealized holding losses, less than 12 months	(119,634)	(568,241)
Available-for-sale securities, estimated fair value, 12 months or longer	8,805,736	3,356,273
Available-for-sale securities, gross unrealized holding losses, 12 months or longer	(1,001,104)	(393,911)
Available-for-sale securities, estimated fair value	12,824,028	10,506,416
Available-for-sale securities, gross unrealized holding losses	$ (1,120,738)	$ (962,152)

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