Investments - Schedule of Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
U.S. government and government agencies
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	$ 352	$ 77,698
AFS Unrealized Losses, Less than 12 months	(36)	(188)
AFS Fair Value, 12 months or longer	110,409	108,578
AFS Unrealized Losses, 12 months or longer	(14,545)	(18,073)
AFS Fair Value	110,761	186,276
AFS Unrealized Losses	(14,581)	(18,261)
Foreign government
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	0	1,552
AFS Unrealized Losses, Less than 12 months	0	(87)
AFS Fair Value, 12 months or longer	9,662	8,251
AFS Unrealized Losses, 12 months or longer	(985)	(1,215)
AFS Fair Value	9,662	9,803
AFS Unrealized Losses	(985)	(1,302)
Obligations of states and political subdivisions
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	49,972	137,031
AFS Unrealized Losses, Less than 12 months	(322)	(962)
AFS Fair Value, 12 months or longer	264,599	290,964
AFS Unrealized Losses, 12 months or longer	(21,301)	(27,965)
AFS Fair Value	314,571	427,995
AFS Unrealized Losses	(21,623)	(28,927)
Corporate securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	164,625	263,423
AFS Unrealized Losses, Less than 12 months	(1,246)	(6,369)
AFS Fair Value, 12 months or longer	1,158,745	1,439,422
AFS Unrealized Losses, 12 months or longer	(85,772)	(131,519)
AFS Fair Value	1,323,370	1,702,845
AFS Unrealized Losses	(87,018)	(137,888)
CLO and other ABS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	264,653	278,940
AFS Unrealized Losses, Less than 12 months	(5,352)	(7,120)
AFS Fair Value, 12 months or longer	657,379	984,175
AFS Unrealized Losses, 12 months or longer	(43,318)	(78,885)
AFS Fair Value	922,032	1,263,115
AFS Unrealized Losses	(48,670)	(86,005)
RMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	233,580	351,976
AFS Unrealized Losses, Less than 12 months	(966)	(4,765)
AFS Fair Value, 12 months or longer	727,552	757,914
AFS Unrealized Losses, 12 months or longer	(62,595)	(81,086)
AFS Fair Value	961,132	1,109,890
AFS Unrealized Losses	(63,561)	(85,851)
CMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	27,946	130,189
AFS Unrealized Losses, Less than 12 months	(107)	(2,995)
AFS Fair Value, 12 months or longer	451,713	471,256
AFS Unrealized Losses, 12 months or longer	(23,403)	(42,135)
AFS Fair Value	479,659	601,445
AFS Unrealized Losses	(23,510)	(45,130)
Available-for-Sale Securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	741,128	1,240,809
AFS Unrealized Losses, Less than 12 months	(8,029)	(22,486)
AFS Fair Value, 12 months or longer	3,380,059	4,060,560
AFS Unrealized Losses, 12 months or longer	(251,919)	(380,878)
AFS Fair Value	4,121,187	5,301,369
AFS Unrealized Losses	$ (259,948)	$ (403,364)

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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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