Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 290,711	$ 381,388
Unrealized losses, less than 12 months	(1,767)	(4,567)
Fair value, 12 months or greater	3,083,647	3,981,258
Unrealized losses, 12 months or greater	(343,381)	(438,964)
Fair value, total	3,374,358	4,362,646
Unrealized losses, total	(345,148)	(443,531)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	5,103	15,650
Unrealized losses, less than 12 months	(10)	(938)
Fair value, 12 months or greater	120,876	113,678
Unrealized losses, 12 months or greater	(24,624)	(25,953)
Fair value, total	125,979	129,328
Unrealized losses, total	(24,634)	(26,891)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	17,690	17,154
Unrealized losses, less than 12 months	(432)	(551)
Fair value, 12 months or greater	86,609	104,949
Unrealized losses, 12 months or greater	(12,051)	(15,486)
Fair value, total	104,299	122,103
Unrealized losses, total	(12,483)	(16,037)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	135,130	161,924
Unrealized losses, less than 12 months	(835)	(1,261)
Fair value, 12 months or greater	1,700,938	2,055,113
Unrealized losses, 12 months or greater	(209,467)	(257,916)
Fair value, total	1,836,068	2,217,037
Unrealized losses, total	(210,302)	(259,177)
RMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	35,229	113,506
Unrealized losses, less than 12 months	(199)	(1,439)
Fair value, 12 months or greater	606,366	653,955
Unrealized losses, 12 months or greater	(67,461)	(82,659)
Fair value, total	641,595	767,461
Unrealized losses, total	(67,660)	(84,098)
CMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	12,822	8,558
Unrealized losses, less than 12 months	(38)	(31)
Fair value, 12 months or greater	449,697	546,104
Unrealized losses, 12 months or greater	(26,182)	(46,666)
Fair value, total	462,519	554,662
Unrealized losses, total	(26,220)	(46,697)
CLO
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	57,047	15,083
Unrealized losses, less than 12 months	(120)	(25)
Fair value, 12 months or greater	72,100	438,294
Unrealized losses, 12 months or greater	(1,711)	(6,528)
Fair value, total	129,147	453,377
Unrealized losses, total	(1,831)	(6,553)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	27,690	49,513
Unrealized losses, less than 12 months	(133)	(322)
Fair value, 12 months or greater	47,061	64,078
Unrealized losses, 12 months or greater	(1,885)	(3,715)
Fair value, total	74,751	113,591
Unrealized losses, total	$ (2,018)	(4,037)
Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months		0
Unrealized losses, less than 12 months		0
Fair value, 12 months or greater		5,087
Unrealized losses, 12 months or greater		(41)
Fair value, total		5,087
Unrealized losses, total		$ (41)