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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	$ 14,932,286	$ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains	101,335	13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses	(660,889)	(962,152)
Available-for-sale fixed maturity investments, estimated fair value	14,372,732	11,856,835
Short-term investments, available-for-sale, amortized cost basis	2,564,620	2,663,560
Short-term investments, available-for-sale, accumulated gross unrealized gain before tax	7,155	5,760
Short-term investments, available-for-sale, accumulated gross unrealized loss before tax	(393)	(58)
Short-term investments, available-for-sale	2,571,382	2,669,262
Investments, available-for-sale, amortized cost basis	17,496,906	15,469,447
Investments, available-for-sale, accumulated gross unrealized gain before tax	108,490	18,860
Investments, available-for-sale, accumulated gross unrealized loss before tax	(661,282)	(962,210)
Investments, available-for-sale	16,944,114	14,526,097
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	3,811,293	3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains	35,824	2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses	(62,404)	(138,493)
Available-for-sale fixed maturity investments, estimated fair value	3,784,713	2,913,959
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	1,225,426	871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains	7,292	154
Available-for-sale fixed maturity investments, gross unrealized holding losses	(89,904)	(106,079)
Available-for-sale fixed maturity investments, estimated fair value	1,142,814	765,538
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	4,196,096	3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains	14,787	6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses	(181,578)	(247,231)
Available-for-sale fixed maturity investments, estimated fair value	4,029,305	3,733,183
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	1,858,845	1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains	21,450	2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses	(96,874)	(169,723)
Available-for-sale fixed maturity investments, estimated fair value	1,783,421	1,306,778
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	2,371,406	2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains	8,605	395
Available-for-sale fixed maturity investments, gross unrealized holding losses	(136,353)	(169,668)
Available-for-sale fixed maturity investments, estimated fair value	2,243,658	1,940,448
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	491,949	553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains	334	6
Available-for-sale fixed maturity investments, gross unrealized holding losses	(21,861)	(26,804)
Available-for-sale fixed maturity investments, estimated fair value	470,422	526,793
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, amortized cost	977,271	773,454
Available-for-sale fixed maturity investments, gross unrealized holding gains	13,043	836
Available-for-sale fixed maturity investments, gross unrealized holding losses	(71,915)	(104,154)
Available-for-sale fixed maturity investments, estimated fair value	$ 918,399	$ 670,136

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