Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Schedule of Investments [Line Items]
Less than 12 months	$ 1,568,486	$ 694,898
12 months or greater	2,674,209	3,664,661
Total	4,242,695	4,359,559
Less than 12 months	(23,848)	(12,254)
12 months or greater	(322,308)	(353,890)
Total	$ (346,156)	$ (366,144)
Number of securities in unrealized loss position | security	1,020	1,021
U.S. government securities
Schedule of Investments [Line Items]
Less than 12 months	$ 37,017	$ 26,550
12 months or greater	69,959	98,359
Total	106,976	124,909
Less than 12 months	(437)	(75)
12 months or greater	(4,650)	(6,289)
Total	(5,087)	(6,364)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less than 12 months	409,406	275,727
12 months or greater	1,195,869	1,200,533
Total	1,605,275	1,476,260
Less than 12 months	(5,621)	(3,622)
12 months or greater	(187,168)	(186,213)
Total	(192,789)	(189,835)
Corporate debt securities
Schedule of Investments [Line Items]
Less than 12 months	852,752	270,956
12 months or greater	1,051,862	1,604,021
Total	1,904,614	1,874,977
Less than 12 months	(10,334)	(6,060)
12 months or greater	(102,505)	(122,151)
Total	(112,839)	(128,211)
ABS
Schedule of Investments [Line Items]
Less than 12 months	20,090	41,549
12 months or greater	49,640	62,611
Total	69,730	104,160
Less than 12 months	(184)	(1,234)
12 months or greater	(1,050)	(2,041)
Total	(1,234)	(3,275)
RMBS
Schedule of Investments [Line Items]
Less than 12 months	171,654	44,867
12 months or greater	151,893	176,349
Total	323,547	221,216
Less than 12 months	(5,498)	(872)
12 months or greater	(20,030)	(19,784)
Total	(25,528)	(20,656)
CMBS
Schedule of Investments [Line Items]
Less than 12 months	77,567	35,249
12 months or greater	151,188	244,216
Total	228,755	279,465
Less than 12 months	(1,774)	(391)
12 months or greater	(6,216)	(15,361)
Total	(7,990)	(15,752)
CLOs
Schedule of Investments [Line Items]
Less than 12 months		0
12 months or greater		274,729
Total		274,729
Less than 12 months		0
12 months or greater		(1,408)
Total		(1,408)
Foreign government debt
Schedule of Investments [Line Items]
Less than 12 months	0	0
12 months or greater	3,798	3,843
Total	3,798	3,843
Less than 12 months	0	0
12 months or greater	(689)	(643)
Total	$ (689)	$ (643)