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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	$ 14,067,742	$ 12,805,887
Available-for-sale fixed maturity investments, gross unrealized holding gains	4,275	13,100
Available-for-sale fixed maturity investments, gross unrealized holding losses	(1,120,738)	(962,152)
Available-for-sale fixed maturity investments, estimated fair value	12,951,279	11,856,835
Short-term investments, available-for-sale, amortized cost	2,250,219	2,663,560
Short-term investments, available-for-sale, gross unrealized holding gains	3,253	5,760
Short-term investments, available-for-sale, gross unrealized holding losses	(746)	(58)
Short-term investments, available-for-sale, estimated fair value	2,252,726	2,669,262
Investments, available-for-sale, amortized cost	16,317,961	15,469,447
Investments, available-for-sale, gross unrealized holding gains	7,528	18,860
Investments, available-for-sale, gross unrealized holding losses	(1,121,484)	(962,210)
Investments, available-for-sale, estimated fair value	15,204,005	14,526,097
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	3,440,991	3,050,089
Available-for-sale fixed maturity investments, gross unrealized holding gains	0	2,363
Available-for-sale fixed maturity investments, gross unrealized holding losses	(130,162)	(138,493)
Available-for-sale fixed maturity investments, estimated fair value	3,310,829	2,913,959
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,133,417	871,463
Available-for-sale fixed maturity investments, gross unrealized holding gains	0	154
Available-for-sale fixed maturity investments, gross unrealized holding losses	(137,733)	(106,079)
Available-for-sale fixed maturity investments, estimated fair value	995,684	765,538
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	4,106,251	3,973,911
Available-for-sale fixed maturity investments, gross unrealized holding gains	879	6,503
Available-for-sale fixed maturity investments, gross unrealized holding losses	(308,184)	(247,231)
Available-for-sale fixed maturity investments, estimated fair value	3,798,946	3,733,183
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,749,621	1,473,658
Available-for-sale fixed maturity investments, gross unrealized holding gains	2,626	2,843
Available-for-sale fixed maturity investments, gross unrealized holding losses	(189,128)	(169,723)
Available-for-sale fixed maturity investments, estimated fair value	1,563,119	1,306,778
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	2,297,828	2,109,721
Available-for-sale fixed maturity investments, gross unrealized holding gains	11	395
Available-for-sale fixed maturity investments, gross unrealized holding losses	(208,466)	(169,668)
Available-for-sale fixed maturity investments, estimated fair value	2,089,373	1,940,448
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	511,525	553,591
Available-for-sale fixed maturity investments, gross unrealized holding gains	1	6
Available-for-sale fixed maturity investments, gross unrealized holding losses	(32,686)	(26,804)
Available-for-sale fixed maturity investments, estimated fair value	478,840	526,793
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	1,226	1,693
Available-for-sale fixed maturity investments, gross unrealized holding gains	0	0
Available-for-sale fixed maturity investments, gross unrealized holding losses	(28)	(53)
Available-for-sale fixed maturity investments, estimated fair value	1,198	1,640
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, amortized cost	826,883	771,761
Available-for-sale fixed maturity investments, gross unrealized holding gains	758	836
Available-for-sale fixed maturity investments, gross unrealized holding losses	(114,351)	(104,101)
Available-for-sale fixed maturity investments, estimated fair value	$ 713,290	$ 668,496

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