Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Schedule of Investments [Line Items]
Less than 12 months	$ 694,898	$ 3,411,256
12 months or greater	3,664,661	1,592,040
Total	4,359,559	5,003,296
Less than 12 months	(12,254)	(298,637)
12 months or greater	(353,890)	(224,403)
Total	$ (366,144)	$ (523,040)
Number of securities in unrealized loss position | security	1,021	1,226
U.S. government securities
Schedule of Investments [Line Items]
Less than 12 months	$ 26,550	$ 67,531
12 months or greater	98,359	76,246
Total	124,909	143,777
Less than 12 months	(75)	(3,583)
12 months or greater	(6,289)	(6,100)
Total	(6,364)	(9,683)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less than 12 months	275,727	1,344,272
12 months or greater	1,200,533	360,956
Total	1,476,260	1,705,228
Less than 12 months	(3,622)	(157,903)
12 months or greater	(186,213)	(98,170)
Total	(189,835)	(256,073)
Corporate debt securities
Schedule of Investments [Line Items]
Less than 12 months	270,956	1,488,255
12 months or greater	1,604,021	758,732
Total	1,874,977	2,246,987
Less than 12 months	(6,060)	(109,976)
12 months or greater	(122,151)	(86,401)
Total	(128,211)	(196,377)
ABS
Schedule of Investments [Line Items]
Less than 12 months	41,549	53,201
12 months or greater	62,611	67,073
Total	104,160	120,274
Less than 12 months	(1,234)	(1,008)
12 months or greater	(2,041)	(5,033)
Total	(3,275)	(6,041)
RMBS
Schedule of Investments [Line Items]
Less than 12 months	44,867	77,563
12 months or greater	176,349	136,179
Total	221,216	213,742
Less than 12 months	(872)	(8,572)
12 months or greater	(19,784)	(17,172)
Total	(20,656)	(25,744)
CMBS
Schedule of Investments [Line Items]
Less than 12 months	35,249	166,973
12 months or greater	244,216	70,792
Total	279,465	237,765
Less than 12 months	(391)	(12,951)
12 months or greater	(15,361)	(7,640)
Total	(15,752)	(20,591)
CLOs
Schedule of Investments [Line Items]
Less than 12 months	0	213,461
12 months or greater	274,729	114,459
Total	274,729	327,920
Less than 12 months	0	(4,644)
12 months or greater	(1,408)	(3,185)
Total	(1,408)	(7,829)
Foreign government debt
Schedule of Investments [Line Items]
Less than 12 months	0	0
12 months or greater	3,843	3,787
Total	3,843	3,787
Less than 12 months	0	0
12 months or greater	(643)	(699)
Total	$ (643)	(699)
Commercial paper
Schedule of Investments [Line Items]
Less than 12 months		0
12 months or greater		3,816
Total		3,816
Less than 12 months		0
12 months or greater		(3)
Total		$ (3)