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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	$ 15,721,795	$ 14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains	27,435	101,335
Available-for-sale fixed maturity investments, gross unrealized holding losses	(771,716)	(660,889)
Available-for-sale fixed maturity investments, estimated fair value	14,977,514	14,372,732
Short-term investments, available-for-sale, amortized cost	2,788,636	2,564,620
Short-term investments, available-for-sale, gross unrealized holding gains	531	7,155
Short-term investments, available-for-sale, gross unrealized holding losses	(1,409)	(393)
Short-term investments, available-for-sale, estimated fair value	2,787,758	2,571,382
Investments, available-for-sale, amortized cost	18,510,431	17,496,906
Investments, available-for-sale, gross unrealized holding gains	27,966	108,490
Investments, available-for-sale, gross unrealized holding losses	(773,125)	(661,282)
Investments, available-for-sale, estimated fair value	17,765,272	16,944,114
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	4,600,441	3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains	7,250	35,824
Available-for-sale fixed maturity investments, gross unrealized holding losses	(83,294)	(62,404)
Available-for-sale fixed maturity investments, estimated fair value	4,524,397	3,784,713
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,347,637	1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains	3,567	7,292
Available-for-sale fixed maturity investments, gross unrealized holding losses	(103,335)	(89,904)
Available-for-sale fixed maturity investments, estimated fair value	1,247,869	1,142,814
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	4,000,057	4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains	6,859	14,787
Available-for-sale fixed maturity investments, gross unrealized holding losses	(214,490)	(181,578)
Available-for-sale fixed maturity investments, estimated fair value	3,792,426	4,029,305
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,984,422	1,858,845
Available-for-sale fixed maturity investments, gross unrealized holding gains	1,915	21,450
Available-for-sale fixed maturity investments, gross unrealized holding losses	(138,458)	(96,874)
Available-for-sale fixed maturity investments, estimated fair value	1,847,879	1,783,421
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	2,325,529	2,371,406
Available-for-sale fixed maturity investments, gross unrealized holding gains	3,433	8,605
Available-for-sale fixed maturity investments, gross unrealized holding losses	(129,334)	(136,353)
Available-for-sale fixed maturity investments, estimated fair value	2,199,628	2,243,658
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	433,954	491,949
Available-for-sale fixed maturity investments, gross unrealized holding gains	143	334
Available-for-sale fixed maturity investments, gross unrealized holding losses	(21,647)	(21,861)
Available-for-sale fixed maturity investments, estimated fair value	412,450	470,422
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,029,755	977,271
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,268	13,043
Available-for-sale fixed maturity investments, gross unrealized holding losses	(81,158)	(71,915)
Available-for-sale fixed maturity investments, estimated fair value	$ 952,865	$ 918,399

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