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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	$ 15,227,752	$ 14,932,286
Available-for-sale fixed maturity investments, gross unrealized holding gains	40,835	101,335
Available-for-sale fixed maturity investments, gross unrealized holding losses	(749,217)	(660,889)
Available-for-sale fixed maturity investments, estimated fair value	14,519,370	14,372,732
Short-term investments, available-for-sale, amortized cost	2,618,162	2,564,620
Short-term investments, available-for-sale, gross unrealized holding gains	597	7,155
Short-term investments, available-for-sale, gross unrealized holding losses	(1,117)	(393)
Short-term investments, available-for-sale, estimated fair value	2,617,642	2,571,382
Investments, available-for-sale, amortized cost	17,845,914	17,496,906
Investments, available-for-sale, gross unrealized holding gains	41,432	108,490
Investments, available-for-sale, gross unrealized holding losses	(750,334)	(661,282)
Investments, available-for-sale, estimated fair value	17,137,012	16,944,114
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	4,113,319	3,811,293
Available-for-sale fixed maturity investments, gross unrealized holding gains	12,079	35,824
Available-for-sale fixed maturity investments, gross unrealized holding losses	(79,408)	(62,404)
Available-for-sale fixed maturity investments, estimated fair value	4,045,990	3,784,713
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,289,119	1,225,426
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,206	7,292
Available-for-sale fixed maturity investments, gross unrealized holding losses	(99,430)	(89,904)
Available-for-sale fixed maturity investments, estimated fair value	1,193,895	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	4,041,845	4,196,096
Available-for-sale fixed maturity investments, gross unrealized holding gains	10,059	14,787
Available-for-sale fixed maturity investments, gross unrealized holding losses	(205,316)	(181,578)
Available-for-sale fixed maturity investments, estimated fair value	3,846,588	4,029,305
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,950,839	1,858,845
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,132	21,450
Available-for-sale fixed maturity investments, gross unrealized holding losses	(126,930)	(96,874)
Available-for-sale fixed maturity investments, estimated fair value	1,828,041	1,783,421
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	2,400,848	2,371,406
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,780	8,605
Available-for-sale fixed maturity investments, gross unrealized holding losses	(137,472)	(136,353)
Available-for-sale fixed maturity investments, estimated fair value	2,268,156	2,243,658
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	463,710	491,949
Available-for-sale fixed maturity investments, gross unrealized holding gains	155	334
Available-for-sale fixed maturity investments, gross unrealized holding losses	(22,187)	(21,861)
Available-for-sale fixed maturity investments, estimated fair value	441,678	470,422
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	968,072	977,271
Available-for-sale fixed maturity investments, gross unrealized holding gains	5,424	13,043
Available-for-sale fixed maturity investments, gross unrealized holding losses	(78,474)	(71,915)
Available-for-sale fixed maturity investments, estimated fair value	$ 895,022	$ 918,399

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