Investments - Schedule of Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities by Length of Time (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	$ 6,334,110	$ 1,121,478
AFS Unrealized Losses, Less than 12 months	(264,150)	(13,646)
AFS Fair Value, 12 months or longer	1,872,321	2,619,304
AFS Unrealized Losses, 12 months or longer	(282,277)	(208,472)
AFS Fair Value	8,206,431	3,740,782
AFS Unrealized Losses	(546,427)	(222,118)
U.S. government and government agencies
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	93,042	28,710
AFS Unrealized Losses, Less than 12 months	(5,474)	(57)
AFS Fair Value, 12 months or longer	89,010	110,826
AFS Unrealized Losses, 12 months or longer	(20,184)	(14,721)
AFS Fair Value	182,052	139,536
AFS Unrealized Losses	(25,658)	(14,778)
Foreign government
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	6,897	0
AFS Unrealized Losses, Less than 12 months	(544)	0
AFS Fair Value, 12 months or longer	7,561	9,058
AFS Unrealized Losses, 12 months or longer	(1,135)	(797)
AFS Fair Value	14,458	9,058
AFS Unrealized Losses	(1,679)	(797)
Obligations of states and political subdivisions
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	302,778	53,076
AFS Unrealized Losses, Less than 12 months	(14,193)	(604)
AFS Fair Value, 12 months or longer	161,130	230,441
AFS Unrealized Losses, 12 months or longer	(34,014)	(23,279)
AFS Fair Value	463,908	283,517
AFS Unrealized Losses	(48,207)	(23,883)
Corporate securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	2,466,362	128,218
AFS Unrealized Losses, Less than 12 months	(102,586)	(3,070)
AFS Fair Value, 12 months or longer	572,314	830,001
AFS Unrealized Losses, 12 months or longer	(87,106)	(68,792)
AFS Fair Value	3,038,676	958,219
AFS Unrealized Losses	(189,692)	(71,862)
CLO and other ABS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	1,421,402	573,832
AFS Unrealized Losses, Less than 12 months	(40,216)	(6,993)
AFS Fair Value, 12 months or longer	328,580	462,469
AFS Unrealized Losses, 12 months or longer	(30,686)	(27,866)
AFS Fair Value	1,749,982	1,036,301
AFS Unrealized Losses	(70,902)	(34,859)
RMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	1,677,296	283,926
AFS Unrealized Losses, Less than 12 months	(91,274)	(1,913)
AFS Fair Value, 12 months or longer	523,639	672,455
AFS Unrealized Losses, 12 months or longer	(93,197)	(59,421)
AFS Fair Value	2,200,935	956,381
AFS Unrealized Losses	(184,471)	(61,334)
CMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	366,333	53,716
AFS Unrealized Losses, Less than 12 months	(9,863)	(1,009)
AFS Fair Value, 12 months or longer	190,087	304,054
AFS Unrealized Losses, 12 months or longer	(15,955)	(13,596)
AFS Fair Value	556,420	357,770
AFS Unrealized Losses	$ (25,818)	$ (14,605)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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