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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	$ 13,653,557	$ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains	7,934	13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses	(915,799)	(962,152)
Available-for-sale fixed maturity investments, estimated fair value	12,745,692	11,856,835
Short-term investments, available-for-sale, amortized cost	2,036,747	2,663,560
Short-term investments, available-for-sale, gross unrealized holding gains	3,840	5,760
Short-term investments, available-for-sale, gross unrealized holding losses	(1,526)	(58)
Short-term investments, available-for-sale, estimated fair value	2,039,061	2,669,262
Investments, available-for-sale, amortized cost	15,690,304	15,469,447
Investments, available-for-sale, gross unrealized holding gains	11,774	18,860
Investments, available-for-sale, gross unrealized holding losses	(917,325)	(962,210)
Investments, available-for-sale, estimated fair value	14,784,753	14,526,097
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	3,262,114	3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains	280	2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses	(133,542)	(138,493)
Available-for-sale fixed maturity investments, estimated fair value	3,128,852	2,913,959
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,044,843	871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains	104	154
Available-for-sale fixed maturity investments, gross unrealized holding losses	(106,212)	(106,079)
Available-for-sale fixed maturity investments, estimated fair value	938,735	765,538
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	4,093,715	3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains	2,728	6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses	(220,108)	(247,231)
Available-for-sale fixed maturity investments, estimated fair value	3,876,335	3,733,183
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,691,459	1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains	3,717	2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses	(151,550)	(169,723)
Available-for-sale fixed maturity investments, estimated fair value	1,543,626	1,306,778
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	2,251,522	2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains	291	395
Available-for-sale fixed maturity investments, gross unrealized holding losses	(178,257)	(169,668)
Available-for-sale fixed maturity investments, estimated fair value	2,073,556	1,940,448
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	508,692	553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains	3	6
Available-for-sale fixed maturity investments, gross unrealized holding losses	(28,773)	(26,804)
Available-for-sale fixed maturity investments, estimated fair value	479,922	526,793
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,241	1,693
Available-for-sale fixed maturity investments, gross unrealized holding gains	0	0
Available-for-sale fixed maturity investments, gross unrealized holding losses	(37)	(53)
Available-for-sale fixed maturity investments, estimated fair value	1,204	1,640
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	799,971	771,761
Available-for-sale fixed maturity investments, gross unrealized holding gains	811	836
Available-for-sale fixed maturity investments, gross unrealized holding losses	(97,320)	(104,101)
Available-for-sale fixed maturity investments, estimated fair value	$ 703,462	$ 668,496

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