Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Mar. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 742,867	$ 381,388
Unrealized losses, less than 12 months	(11,215)	(4,567)
Fair value, 12 months or greater	3,504,994	3,981,258
Unrealized losses, 12 months or greater	(467,080)	(438,964)
Fair value, total	4,247,861	4,362,646
Unrealized losses, total	$ (478,295)	$ (443,531)
Number of securities in unrealized loss position | security	1,041	1,063
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	$ 21,017	$ 15,650
Unrealized losses, less than 12 months	(1,915)	(938)
Fair value, 12 months or greater	107,278	113,678
Unrealized losses, 12 months or greater	(28,096)	(25,953)
Fair value, total	128,295	129,328
Unrealized losses, total	(30,011)	(26,891)
State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	24,066	17,154
Unrealized losses, less than 12 months	(720)	(551)
Fair value, 12 months or greater	99,220	104,949
Unrealized losses, 12 months or greater	(14,817)	(15,486)
Fair value, total	123,286	122,103
Unrealized losses, total	(15,537)	(16,037)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	318,330	161,924
Unrealized losses, less than 12 months	(4,319)	(1,261)
Fair value, 12 months or greater	1,942,216	2,055,113
Unrealized losses, 12 months or greater	(279,256)	(257,916)
Fair value, total	2,260,546	2,217,037
Unrealized losses, total	(283,575)	(259,177)
RMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	129,902	113,506
Unrealized losses, less than 12 months	(3,132)	(1,439)
Fair value, 12 months or greater	635,067	653,955
Unrealized losses, 12 months or greater	(95,227)	(82,659)
Fair value, total	764,969	767,461
Unrealized losses, total	(98,359)	(84,098)
CMBS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	612	8,558
Unrealized losses, less than 12 months	(1)	(31)
Fair value, 12 months or greater	513,908	546,104
Unrealized losses, 12 months or greater	(42,423)	(46,666)
Fair value, total	514,520	554,662
Unrealized losses, total	(42,424)	(46,697)
CLO
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	164,220	15,083
Unrealized losses, less than 12 months	(398)	(25)
Fair value, 12 months or greater	148,248	438,294
Unrealized losses, 12 months or greater	(3,949)	(6,528)
Fair value, total	312,468	453,377
Unrealized losses, total	(4,347)	(6,553)
Other ABS
Debt Securities, Available-for-sale [Line Items]
Fair value, less than 12 months	84,720	49,513
Unrealized losses, less than 12 months	(730)	(322)
Fair value, 12 months or greater	59,057	64,078
Unrealized losses, 12 months or greater	(3,312)	(3,715)
Fair value, total	143,777	113,591
Unrealized losses, total	$ (4,042)	(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)