Investments - Schedule of Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities by Length of Time (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	$ 915,509	$ 2,101,944
AFS Unrealized Losses, Less than 12 months	(15,292)	(45,680)
AFS Fair Value, 12 months or longer	2,714,827	3,141,858
AFS Unrealized Losses, 12 months or longer	(223,148)	(324,401)
AFS Fair Value	3,630,336	5,243,802
AFS Unrealized Losses	(238,440)	(370,081)
U.S. government and government agencies
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	14,800	14,708
AFS Unrealized Losses, Less than 12 months	(200)	(70)
AFS Fair Value, 12 months or longer	110,360	105,326
AFS Unrealized Losses, 12 months or longer	(15,129)	(19,683)
AFS Fair Value	125,160	120,034
AFS Unrealized Losses	(15,329)	(19,753)
Foreign government
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	0	0
AFS Unrealized Losses, Less than 12 months	0	0
AFS Fair Value, 12 months or longer	9,040	9,302
AFS Unrealized Losses, 12 months or longer	(829)	(1,333)
AFS Fair Value	9,040	9,302
AFS Unrealized Losses	(829)	(1,333)
Obligations of states and political subdivisions
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	70,170	153,996
AFS Unrealized Losses, Less than 12 months	(1,158)	(3,539)
AFS Fair Value, 12 months or longer	225,951	247,735
AFS Unrealized Losses, 12 months or longer	(24,428)	(28,820)
AFS Fair Value	296,121	401,731
AFS Unrealized Losses	(25,586)	(32,359)
Corporate securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	186,603	684,999
AFS Unrealized Losses, Less than 12 months	(3,200)	(11,699)
AFS Fair Value, 12 months or longer	841,224	1,083,392
AFS Unrealized Losses, 12 months or longer	(70,530)	(111,502)
AFS Fair Value	1,027,827	1,768,391
AFS Unrealized Losses	(73,730)	(123,201)
CLO and other ABS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	312,803	349,786
AFS Unrealized Losses, Less than 12 months	(6,383)	(6,296)
AFS Fair Value, 12 months or longer	491,315	601,057
AFS Unrealized Losses, 12 months or longer	(30,606)	(43,393)
AFS Fair Value	804,118	950,843
AFS Unrealized Losses	(36,989)	(49,689)
RMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	284,520	714,061
AFS Unrealized Losses, Less than 12 months	(2,875)	(21,206)
AFS Fair Value, 12 months or longer	710,469	677,574
AFS Unrealized Losses, 12 months or longer	(65,845)	(91,516)
AFS Fair Value	994,989	1,391,635
AFS Unrealized Losses	(68,720)	(112,722)
CMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	46,613	184,394
AFS Unrealized Losses, Less than 12 months	(1,476)	(2,870)
AFS Fair Value, 12 months or longer	326,468	417,472
AFS Unrealized Losses, 12 months or longer	(15,781)	(28,154)
AFS Fair Value	373,081	601,866
AFS Unrealized Losses	$ (17,257)	$ (31,024)

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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