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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	$ 16,265,743	$ 14,932,286
Available-for-sale fixed maturity securities, gross unrealized holding gains	239,853	101,335
Available-for-sale fixed maturity securities, gross unrealized holding losses	(437,748)	(660,889)
Available-for-sale fixed maturity securities, estimated fair value	16,067,848	14,372,732
Short-term investments, available-for-sale, amortized cost	2,351,758	2,564,620
Short-term investments, available-for-sale, gross unrealized holding gains	4,070	7,155
Short-term investments, available-for-sale, gross unrealized holding losses	(90)	(393)
Short-term investments, available-for-sale, estimated fair value	2,355,738	2,571,382
Investments, available-for-sale, amortized cost	18,617,501	17,496,906
Investments, available-for-sale, gross unrealized holding gains	243,923	108,490
Investments, available-for-sale, gross unrealized holding losses	(437,838)	(661,282)
Investments, available-for-sale, estimated fair value	18,423,586	16,944,114
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	4,946,495	3,811,293
Available-for-sale fixed maturity securities, gross unrealized holding gains	79,659	35,824
Available-for-sale fixed maturity securities, gross unrealized holding losses	(31,894)	(62,404)
Available-for-sale fixed maturity securities, estimated fair value	4,994,260	3,784,713
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	1,420,858	1,225,426
Available-for-sale fixed maturity securities, gross unrealized holding gains	19,757	7,292
Available-for-sale fixed maturity securities, gross unrealized holding losses	(65,452)	(89,904)
Available-for-sale fixed maturity securities, estimated fair value	1,375,163	1,142,814
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	4,003,008	4,196,096
Available-for-sale fixed maturity securities, gross unrealized holding gains	36,672	14,787
Available-for-sale fixed maturity securities, gross unrealized holding losses	(129,328)	(181,578)
Available-for-sale fixed maturity securities, estimated fair value	3,910,352	4,029,305
Foreign governments, agencies and supranationals [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	2,783,826	2,554,874
Available-for-sale fixed maturity securities, gross unrealized holding gains	77,781	34,352
Available-for-sale fixed maturity securities, gross unrealized holding losses	(103,644)	(142,008)
Available-for-sale fixed maturity securities, estimated fair value	2,757,963	2,447,218
Agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	2,738,131	2,507,682
Available-for-sale fixed maturity securities, gross unrealized holding gains	25,754	8,939
Available-for-sale fixed maturity securities, gross unrealized holding losses	(86,205)	(135,611)
Available-for-sale fixed maturity securities, estimated fair value	2,677,680	2,381,010
Non-agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	122,640	355,673
Available-for-sale fixed maturity securities, gross unrealized holding gains	0	0
Available-for-sale fixed maturity securities, gross unrealized holding losses	(3,416)	(22,603)
Available-for-sale fixed maturity securities, estimated fair value	119,224	333,070
Corporate and university bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, available-for-sale, amortized cost	250,785	281,242
Available-for-sale fixed maturity securities, gross unrealized holding gains	230	141
Available-for-sale fixed maturity securities, gross unrealized holding losses	(17,809)	(26,781)
Available-for-sale fixed maturity securities, estimated fair value	$ 233,206	$ 254,602

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