Investments - Fair Value and Gross Pre-Tax Net Unrealized Loss of Securities By Length of Time (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 8,149	$ 77,698
AFS Unrealized Losses, Less than 12 months	(49)	(188)
AFS Fair Value, 12 months or longer	107,151	108,578
AFS Unrealized Losses, 12 months or longer	(19,530)	(18,073)
AFS Fair Value	115,300	186,276
AFS Unrealized Losses	(19,579)	(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	9,229	8,251
AFS Unrealized Losses, 12 months or longer	(1,445)	(1,215)
AFS Fair Value	9,229	9,803
AFS Unrealized Losses	(1,445)	(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	161,084	137,031
AFS Unrealized Losses, Less than 12 months	(1,289)	(962)
AFS Fair Value, 12 months or longer	303,259	290,964
AFS Unrealized Losses, 12 months or longer	(30,569)	(27,965)
AFS Fair Value	464,343	427,995
AFS Unrealized Losses	(31,858)	(28,927)
Corporate securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	316,330	263,423
AFS Unrealized Losses, Less than 12 months	(4,043)	(6,369)
AFS Fair Value, 12 months or longer	1,446,157	1,439,422
AFS Unrealized Losses, 12 months or longer	(136,142)	(131,519)
AFS Fair Value	1,762,487	1,702,845
AFS Unrealized Losses	(140,185)	(137,888)
CLO and other ABS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	385,471	278,940
AFS Unrealized Losses, Less than 12 months	(7,527)	(7,120)
AFS Fair Value, 12 months or longer	824,843	984,175
AFS Unrealized Losses, 12 months or longer	(66,470)	(78,885)
AFS Fair Value	1,210,314	1,263,115
AFS Unrealized Losses	(73,997)	(86,005)
RMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	511,009	351,976
AFS Unrealized Losses, Less than 12 months	(8,770)	(4,765)
AFS Fair Value, 12 months or longer	761,356	757,914
AFS Unrealized Losses, 12 months or longer	(95,467)	(81,086)
AFS Fair Value	1,272,365	1,109,890
AFS Unrealized Losses	(104,237)	(85,851)
CMBS
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	133,266	130,189
AFS Unrealized Losses, Less than 12 months	(2,033)	(2,995)
AFS Fair Value, 12 months or longer	494,395	471,256
AFS Unrealized Losses, 12 months or longer	(37,679)	(42,135)
AFS Fair Value	627,661	601,445
AFS Unrealized Losses	(39,712)	(45,130)
Available-for-Sale Securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
AFS Fair Value, Less than 12 Months	1,515,309	1,240,809
AFS Unrealized Losses, Less than 12 months	(23,711)	(22,486)
AFS Fair Value, 12 months or longer	3,946,390	4,060,560
AFS Unrealized Losses, 12 months or longer	(387,302)	(380,878)
AFS Fair Value	5,461,699	5,301,369
AFS Unrealized Losses	$ (411,013)	$ (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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