Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Jun. 30, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 827,831	$ 381,388
Unrealized losses, less than 12 months	(11,156)	(4,567)
Fair value, 12 months or greater	3,409,088	3,981,258
Unrealized losses, 12 months or greater	(483,677)	(438,964)
Fair value, total	4,236,919	4,362,646
Unrealized losses, total	$ (494,833)	$ (443,531)
Number of securities in unrealized loss position | security	1,037	1,063
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 6,009	$ 15,650
Unrealized losses, less than 12 months	(349)	(938)
Fair value, 12 months or greater	119,459	113,678
Unrealized losses, 12 months or greater	(31,232)	(25,953)
Fair value, total	125,468	129,328
Unrealized losses, total	(31,581)	(26,891)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	28,122	17,154
Unrealized losses, less than 12 months	(1,216)	(551)
Fair value, 12 months or greater	92,942	104,949
Unrealized losses, 12 months or greater	(15,634)	(15,486)
Fair value, total	121,064	122,103
Unrealized losses, total	(16,850)	(16,037)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	353,929	161,924
Unrealized losses, less than 12 months	(6,424)	(1,261)
Fair value, 12 months or greater	1,872,436	2,055,113
Unrealized losses, 12 months or greater	(290,787)	(257,916)
Fair value, total	2,226,365	2,217,037
Unrealized losses, total	(297,211)	(259,177)
RMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	175,512	113,506
Unrealized losses, less than 12 months	(2,249)	(1,439)
Fair value, 12 months or greater	679,163	653,955
Unrealized losses, 12 months or greater	(102,526)	(82,659)
Fair value, total	854,675	767,461
Unrealized losses, total	(104,775)	(84,098)
CMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	13,429	8,558
Unrealized losses, less than 12 months	(22)	(31)
Fair value, 12 months or greater	479,194	546,104
Unrealized losses, 12 months or greater	(37,364)	(46,666)
Fair value, total	492,623	554,662
Unrealized losses, total	(37,386)	(46,697)
CLO
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	113,018	15,083
Unrealized losses, less than 12 months	(242)	(25)
Fair value, 12 months or greater	98,002	438,294
Unrealized losses, 12 months or greater	(2,380)	(6,528)
Fair value, total	211,020	453,377
Unrealized losses, total	(2,622)	(6,553)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	137,812	49,513
Unrealized losses, less than 12 months	(654)	(322)
Fair value, 12 months or greater	67,892	64,078
Unrealized losses, 12 months or greater	(3,754)	(3,715)
Fair value, total	205,704	113,591
Unrealized losses, total	$ (4,408)	(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)