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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	$ 16,457,723
Available-for-sale fixed maturity investments, estimated fair value	15,745,539	$ 14,372,732
Short-term investments, available-for-sale, amortized cost basis	2,530,941	2,564,620
Short-term investments, available-for-sale, accumulated gross unrealized gain before tax	548	7,155
Short-term investments, available-for-sale, accumulated gross unrealized loss before tax	(6,579)	(393)
Short-term investments, available-for-sale	2,524,910	2,571,382
Investments, available-for-sale, amortized cost basis	18,988,664	17,496,906
Investments, available-for-sale, accumulated gross unrealized gain before tax	24,788	108,490
Investments, available-for-sale, accumulated gross unrealized loss before tax	(743,003)	(661,282)
Investments, available-for-sale	18,270,449	16,944,114
Investments, Available-for-sale, Allowance for Credit Loss	0	0
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	5,147,365	3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains	8,962	35,824
Available-for-sale fixed maturity investments, gross unrealized holding losses	(68,469)	(62,404)
Available-for-sale fixed maturity investments, estimated fair value	5,087,858	3,784,713
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	1,445,171	1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains	2,976	7,292
Available-for-sale fixed maturity investments, gross unrealized holding losses	(101,911)	(89,904)
Available-for-sale fixed maturity investments, estimated fair value	1,346,236	1,142,814
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	3,813,146	4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains	5,866	14,787
Available-for-sale fixed maturity investments, gross unrealized holding losses	(199,520)	(181,578)
Available-for-sale fixed maturity investments, estimated fair value	3,619,492	4,029,305
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	2,909,561	2,554,874
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,264	34,352
Available-for-sale fixed maturity investments, gross unrealized holding losses	(207,302)	(142,008)
Available-for-sale fixed maturity investments, estimated fair value	2,706,523	2,447,218
Agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	2,771,589	2,507,682
Available-for-sale fixed maturity investments, gross unrealized holding gains	2,096	8,939
Available-for-sale fixed maturity investments, gross unrealized holding losses	(123,872)	(135,611)
Available-for-sale fixed maturity investments, estimated fair value	2,649,813	2,381,010
Non-agency mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	122,373	355,673
Available-for-sale fixed maturity investments, gross unrealized holding gains	0	0
Available-for-sale fixed maturity investments, gross unrealized holding losses	(4,343)	(22,603)
Available-for-sale fixed maturity investments, estimated fair value	118,030	333,070
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	248,518	281,242
Available-for-sale fixed maturity investments, gross unrealized holding gains	76	141
Available-for-sale fixed maturity investments, gross unrealized holding losses	(31,007)	(26,781)
Available-for-sale fixed maturity investments, estimated fair value	217,587	254,602
Total fixed maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	16,457,723	14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains	24,240	101,335
Available-for-sale fixed maturity investments, gross unrealized holding losses	(736,424)	(660,889)
Available-for-sale fixed maturity investments, estimated fair value	$ 15,745,539	$ 14,372,732

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